Not since Richard Sears hired Alvah Roebuck and started a mail- order watch business 95 years ago has Sears, Roebuck and Co. made such a far-reaching decision.

As casually as a shopper might pick up a Craftsman screwdriver set one day and go back later for a pair of Cheryl Tiegs jeans, Sears in a single week bought not only the nation's fifth-largest stockbroker but also the biggest independent real estate brokerage.

The planned acquisitions of Dean Witter Reynolds Organization Inc. for $600 million and Coldwell, Banker & Co. for $170 million are a $.75 billion downpayment on a new corporate strategy for Sears.

The world's biggest retailer is determined to become the nation's "largest consumer-oriented financial service entity," vows Sears Chairman Edward Telling.

The company that calls itself "the place America shops" wants also to be the place where America invests--and buys its homes, gets its mortgages, takes out its insurance and manages its money.

Sears is going into the money business with the same mass-market approach that has made it the biggest seller of tools, appliances and clothing.

Other stock brokers may have more assets, but with 330 branches, Dean Witter's retail brokerage network is second only to Merrill Lynch, Pierce Fenner & Smith.

Coldwell, Banker isn't as familiar a name as Kenmore, but the company is at least twice as big as any other in commercial real estate sales. Its residential business--including the local Routh-Robbins firm--is likewise the biggest chain of real estate offices under one ownership. (Most national real estate "chains" like Century 21 and Red Carpet are franchised independents linked by a common name.)

Not even executives of Dean Witter and Coldwell, Banker know yet where Telling's strategy will lead their combined companies, but the possibilities boggle the mind.

"It could be awesome if somebody can put the pieces together," said James O'Brien, who heads Coldwell, Banker's Washington office.

Neither O'Brien's office at 21st and K streets NW nor the Dean Witter branch a couple blocks away is likely to move into the Sears store on Wisconsin Avenue, but some combined marketing efforts are considered certain.

The assortment of financial services the combined companies will be able to offer is as broad as the mix of merchandise in the biggest Sears store.

Consider what Sears is getting and what it already has:

Sears Merchandising Group: sales of $16.8 billion last year, 854 stores, enough catalogs to dwarf the best-seller list.

Allstate Insurance Group: $6.2 billion in 1980 revenues, 20 million policyholders, and insurance for everything from autos to annuities.

Seraco Group: $420 million in revenues for its first full year of operation as a separate entity; Homart Development Co., the third-largest shopping center builder; Allstate Savings and Loan Association, a California financial institution with $3 billion in assets (as big as Riggs National bank); PMI Group, issuer of $9 billion in private mortgage insurance; and Allstate Enterprises, a major mortgage banker.

Preston Martin, the former head of the Federal Home Loan Bank Board who now runs Seraco, said last month any real estate brokerage firm Sears acquired would not necessarily do business through the Sears retail stores.

"I am sure we'll remain separate organizations for a while," said a source close to Dean Witter.

But Sears, Coldwell Banker, Dean Witter and Allstate will be able to share information. Sears' incredible customer base alone is an asset that no other stock broker or real estate agency can match.

Not only does Sears have 25 million card-carrying credit customers, it has twice as many charge accounts among families earning more than $36,000 a year as does American Express, according to an SRI International study.

Computers can easily sort out the deadbeats and the most affluent. A Dean Witter broker would relish knowing which Sears credit card customers spend freely. Or who just bought or sold a house through a Coldwell, Banker agent. Or took out a $100,000 life insurance policy with Allstate.

Allstate agents might find a fruitful list of new clients through Coldwell, Banker or Dean Witter. Allstate's insurance policies already are sold by hundreds of independent agents and lots of stockbrokers offer specialized insurance to clients. Might not Dean Witter put Allstate on its platter of financial services?

Dean Witter could refer investors interested in real estate to Coldwell, Banker or find mortgage investors for Seraco.

Sears a few weeks ago announced plans to start its own money-market fund; Dean Witter already has four of them. Sears might tie its credit card and money-market fund together, connect them to automatic teller machines in every store and set up a nationwide system to compete with Visa, MasterCard or the local bank.

"Let's face it," said one industry analyst, "many individuals are put off by the idea of going to a broker, and aren't even aware that if they have $1,000 they can invest their money at interest yields of 16 or 17 percent. They might not have the $10,000 they need to get a money-market certificate of deposit at a bank, but they may have $1,000 for a Sears money fund."

Coldwell, Banker real estate agents could sell a house, offer home owners' insurance through Allstate, arrange a mortgage through Coldwell, Banker or Seraco and write private mortgage insurance via PMI.

An executive being transferred could sell a house through Sears/Coldwell, Banker, then temporarily park the cash in a Sears/Dean Witter money fund. The whole move might be handled by Executrans, an existing joint venture of Sears and Coldwell, Banker.

Sears' success with its Allstate insurance company has already proven that the nationwide chain of stores can successfully market an intangible product.

The 50-year-old insurance subsidiary long ago outgrew its little booths in the Sears stores and last year earned twice as much profit--$450 million versus $209 million--as all the Sears retail operations.

On sales of $16.8 billion last year, Sears' merchandising group turned a puny profit of only 1.2 percent, a margin as slim as that of supermarkets and a fraction of what many smaller store chains earn.

Sears the retailer is what business students euphemistically call a "mature industry"--an enterprise that's grown as big as it can grow. Most every town big enough to support a Sears store already has one and the company's prospects for increasing its share of U.S. retail sales are limited.

Throughout the '70s, Sears struggled for a bigger slice of the retail pie. For a time, Sears tried to trade up its customers to better goods that carried higher markups. Then it attempted to take on K-mart and capture part of the discount store market. Neither technique worked.

The strategy outlined at the company's annual stockholders meeting last May calls for tightening up the retail operation and seeking growth in other fields.

Sears hopes to capitalize on one of the basic buzzwords of the '60s-- synergy: the ability of two or more organizations working together to produce results exceeding anything they could do separately.

Synergistic promises fell from favor when many conglomerates found they could not manage unrelated enterprises very well, but it is back in the patter of executives talking about mergers in the financial services industry.

If Sears can harness the synergy of its own financial services plus Coldwell, Banker and Dean Witter, it will put itself back on a growth track that it fell off during the 1970s.

Sears is not alone in looking to create an all-things-to-all-customers financial corporation. Merrill Lynch embarked on that path years ago, from a base on Wall Street rather than Main Street.

This year four major Wall Street firms have been absorbed by other companies seeking to create, if not a financial supermarket, at least a well-stocked deli.

Prudential Insurance Co., the nation's biggest insurance company, bought Bache Group Inc., the nation's sixth-biggest brokerage firm. Then American Express Co. bought Shearson Loeb Rhoades, the second-biggest broker. Phibro Corp., the world's largest publicly owned commodities trading company, bought out Salomon Brothers. And an affiliate of Bechtel Corp., the giant engineering firm, bought a large part of the middle-sized investment banking firm Dillon, Read & Co.

But talking about synergy and achieving it are two different things. A top executive at a major brokerage firm admitted that while the concept looks great on paper, individuals do not shop for financial services the way they shop for foodstuffs.

"You don't put life insurance next to mutual funds and put the real estate agent the next aisle over," he said. "It's a personal business. You have troubles devising ways to make sure real estate agents think of your brokers and your brokers think of your real estate agents."

Furthermore, added the chief executive of a major financial services company, many customers may feel threatened by the idea of doing all or most of their financial business with one company. Many people, he noted, are reluctant to have their bank credit cards with the same institution where they have checking accounts.

Nor does Sears itself have a proven ability to change its own direction. Sears' up-scale, down-scale merchandising flight over the past decade never convinced anyone that the company knew where it was going.

After this week's corporate shopping spree, Sears' direction is determined. With assets of $28 billion, Sears can clearly afford the fare. The question now is: Can you get there from here?