DEAR BOB: In spite of high prices and high mortgage interest rates, we've decided to go ahead and buy our first home. We started looking last weekend. But we're confused about the asking prices. It seems that basically similar three-bedroom homes in the same neighborhood vary in price by thousands of dollars. How can we be sure we're not paying too much? Jack M., Alexandria.

DEAR JACK: Your first profit on a home purchase is made when you buy at the home's true market value. Not above. Not below.

So your next question probably is "How do I determine the true market value of a home?" The answer is in two parts.

Forget the sellers' asking prices. Those are their "dream prices." However, if the sellers have been properly counseled by their real estate agents, they may have set the asking prices right at market value - so don't hesitate to pay the full asking price in those few situations.

Before you make a purchase offer on the home you want to buy, insist that your agent prepare a written competitive market analysis. This report to you will show (1) at least six recent home sales in the neighborhood, their sales prices, terms, and special features of the houses, (2) asking prices of similar houses now for sale in the neighborhood, and (c) the agent's suggested offer price, based on the analysis of these similar neighborhood homes.

By the way, the written competitive market analysis has another important use. When your agent presents your purchase offer to the seller, this analysis will be used by the agent to show the seller why your offer is at true market value and why the seller should accept it.

DEAR BOB: We are planning to move in two months. Should we list our home for sale now, with an occupancy date in two months. or should we wait to list after we've moved out? R. C., Forest Heights.

DEAR R.C.: List your home now with occupancy date in two months. It often takes 30 to 90 days to sell a house, plus an additional 30 days or more to complete the sale after arranging new mortgage, title inspection and other closing details.

Another reason to list your home now that is an occupied home looks better than a vacant one. If you're living in the house, prospective buyers aren't likely to make big allowances in their purchase offers for minor defects, such as the need for paint. But a vacant home invites close inspection and lower purchase offers.

DEAR BOB: In 1976 we sold our condo and deferred the profit tax as we bought a "replacement principal residence." When we sell our current condo, can we use that $100,000 "over 55 rule" tax exemption to avoid tax on both our deferred profit and the profit on the sale of our current condo? N.G., Arlington.

DEAR N.G.: Yes. Any person of any age who sells his or her primary residence and buys a more expensive replacement within 18 months before or after the sale must defer paying their profit tax as you did.

When you qualify for the over-55 exemption, you or your co-owner spouse must be 55 or older on the title transfer date; have owned and lived in the principal residence at least three or the five years before sale, and never have used this exemption before. Ask your tax advisor to explain further.

The new Bruss report, "How to Buy a Home for the Lowest Price and on the Best Terms," is available for 25 cents plus a self-addressed, stamped envelope sent to Robert J. Bruss, P.O. 6710, San Francisco, Calif. 94101.