Realtors may be pleased at the prospect of a Ronald Reagan administration, but they recognize that the Republicans will not be able to do anything right away about the high mortgage rates currently stifling the real estate market.

About 10,000 members of the National Association of Realtors, meeting here this week for their annual convention, were told repeatedly that they will face tough times in 1981 because of the economic conditions that continue to shake the foundations of mortgage lending.

Interest rates, now higher than 14 percent, could rise to 16 percent by the end of this year and then start to decline slightly, forecasters said.

The residential market will continue to cool this winter, especially for medium-priced houses, the Realtors were told.

They are reluctant to give up existing mortgages, at fixed rates below 10 percent, for the new short-term financing at high rates being offered by lenders who are increasingly reluctant to make 30-year commitments on loans.

Only FHA- and VA-backed loans, with their 13 percent ceilings, continue to be available for detached houses. However, market conditions now require sellers of houses with these government-backed mortgages to pay more than eight discount points in order for their buyers to qualify.

Participants in the secondary market, such the Federal National Mortgage Association, find themselves holding loans made at 10 percent while being forced to borrow money at 13 percent. FNMA Chairman Oakley Hunter said his corporation has a "big portfolio of existing loans on which the average return is only 9.08 percent."

This contributed to the corporation's loss in the recent quarter and underlies its unpopular proposal to require that existing conventional mortgages be paid off when a house is resold. Lenders, who are now paying higher interest to savers and investors in mortgages, point out that the flip side of being able to perpetuate old, low-rate mortgages is that it forces up borrowing costs for new homebuyers.

To counteract some of the opposition to its due-on-sale proposal, Fannie Mae wants to offer buyers of existing houses a slightly below-market rate in exchange for relinquishing the right to assume a lower existing loan. FNMA's new, renegotiated loan could be significantly higher than the seller's previous mortgage rate, but lower than the Federal Home Loan Bank Board's closed loan index rate -- a rate established each month by the Board.

Buyers can now find out if Fannie Mae has repurchased their loans by asking the original lender.

A house seller who takes back a mortgage can sell that mortgage on the secondary market for the first time, however the seller must go to a lender (bank or savings and loan) approved by Fannie Mae. The seller puts up the loan money, but pays the financial institution a negotiated fee (approximately 1 percent of the loan) to do the paperwork on the loan and collect the monthly mortgage payments over the life of the mortgage. The institution then may sell the loan to Fannie Mae.

The seller must make sure when the deal is closed that the institution plans to sell the loan to Fannie Mae so the seller can expect to get reimbursed by Fannie Mae. The institution must therefore have outstanding commitments with Fannie Mae, or agreements to purchase. The seller, in addition to paying the servicing fees must also sell his loan on the open market, at a discount if rates have gone up. For example, if a seller makes a loan to the buyer at 12 percent and the market rate in the meanwhile goes up to 14 percent, the house seller must accept less than the face value of his loan in order to sell it to Fannie Mae. The reverse would be true if interest rates are going down.