DEAR BOB: What time of year is best to buy property when the prices are the lowest? Francis K., MARYLAND.

DEAR FRANCES: Late fall and winter are often good times to buy when buyer demands is usually low, except in vacation areas like Florida and Arizona, of course. Sellers often get discouraged from lack of buyers in fall and winter and are willing to discount their asking prices then.

Spring brings out the buyers and prices tend to rise as demand increases. Summer can be a good time to buy because many homes come on the market for sale then.

No matter what the season, your key to finding a fairly priced property is to buy from a motivated seller who must sell quickly. Keep looking.

DEAR BOB: A few weeks ago I read with great interest your explanation of how to buy discounted mortgages through mortgage loan brokers and some real estate brokers. But it seems to me that yields of 15 to 20 per cent, and more, would violate the state usury laws. Barry A., Virginia.

DEAR BARRY: I have yet to find a state in which it is usurious to buy an existing mortgage from its owner for less than the balance owed on it by the borrower. That is the simple procedure for buying discounted mortgage.

For example, suppose a property seller took back from his buyer a $10,000 mortgage for 15 years at only 6 per cent interest. He decides to sell it to raise cash. Very few people would buy such a mortgage because they can earn higher interest with more safety on an insured savings account.

To make such a mortgage attractive to a buyer, the mortgage seller must offer it for less than the face amount owed by the borrower. This raises the yield to an investor and allows him to earn a decent profit for his investment risk. It's not uncommon to see mortgage discounts for up to 20 and even 50 per cent if the interest rate is low and the term is long. Such mortgages make excellent investments for ilnvestors seeking good income. But they offer no tax shelter as do many property investments.

For your copy of his report, "How Senior Citizens Can Save Taxes When Selling Their Residences," send 10 cents in coin and a STAMPED self-addressed envelope to P.O. Box 6710, San Francisco, Calif. 94101.