DEAR BOB: I own a rental town house that I can sell for $126,000. Will I be subject to tax on my profit if I reinvest in another investment property such as apartments? Could I reinvest in common stocks without paying profit tax? Jose C., Washington.

DEAR JOSE: Uncle Sam is going to love you if you sell your income property and reinvest the proceeds in another property or anything else. Whenever you make an outright sale of investment property, your profit becomes taxable (as long term capital gain if you owned it over 12 months). There is no way to avoid tax if you sell and reinvest.

However, you can defer the profit tax if you make a tax-deferred property exchange. For example, you could trade your town house for other investment real estate, such as an apartment house, without owing tax.

To qualify, the property acquired must cost more than the value of your property traded. If the other party to the trade doesn't want your town house, it can be sold immediately after your tax-deferred "up trade" is completed. In fact, most trades involve three persons (the two traders and a "cash out buyer"). They are called three-way trades. For more details see your tax advisor or a real estate exchange specialists.

DEAR BOB: We thought we sold our land. But the buyer keeps stalling on the closing with one excuse after another. Would it be legal for us to take another "back-up offer" from a different buyer in case the first one can't close the purchase? James A., Alexandria.

DEAR JAMES: You can take a back-up offer, and it is wise to do so. You can accept the second purchase offer conditioned upon obtaining a release from the first buyer. Once you obtain a written release from the first buyer, you can go ahead with the second sale. Your attorney can guide you further.

DEAR BOB: When do you think interest rates on home mortgages will come down? Albert E., Burke.

DEAR ALBERT: Not soon. There are too many inflationary pressures pushing home mortgages interest rates up. If you're waiting to buy a home, buy it now before buyer demand and limited supply drive prices higher.

DEAR BOB: I am 57 and my wife is 54. We are eligible for that new $100,000 home sale exemption, as we have lived in our home more than three of the last five years. If we sell now, our profit will be about $40,000. As we understand the tax law, we would be wasting the remaining $60,000 if we use the $100,000 tax exemption now. Is this correct? Our plan is to sell now and spend six months traveling overseas. Then we may buy a condominium in a southern state. How can we best save tax? Joel M., Salisbury, Md.

DEAR JOEL: You are correct that the $100,000 home sale profit tax exemption is available only once per lifetime. Any unused portion is wasted.

Although it appears you qualify, as you are 55 or older and have owned and lived in the home at least three of the five years before sale, perhaps you shouldn't use up your $100,000 tax exemption. You may be better off using the "rollover" residence replacement rule which is available to home sellers of any age.

To qualify for this profit tax deferral, you must buy a replacement home costing at least as much as the sales price of your former principal residence. You can buy the replacement anytime within 18 months before or after the sale. If you build a new home, you can take up to 24 months after the sale. Single-family houses, condos, cooperatives, mobile homes, and houseboats can qualify. Your tax advisor has full details.

DEAR BOB: We plan to sell our home this summer. At your suggestion, we have already talked to several realty agents about listing it for sale. Several want 90-day listings but one every pursuasive agent wants a 12-month listing. We checked all the references given by the agents, as you suggest. The former clients of the woman wanting the 12-month listing seem to be best. Her clients rave about her. Do you think a 12-month listing is too long as we must get our home sold by September? Verna G., Fairfax.

DEAR VERNA: A 12-month listing for the sale of a home is far too long. The general rule is the shorter the listing term, the harder, smarter and faster an agent will work to get the home sold before the listing expires.

Although difficult-to-sell property might require a six-month exclusive listing, a 90-day listing is usually sufficient to get a house sold. Offer the agent a 90-day listing with the promise of a renewal if the house isn't sold when the listing expires if the agent is doing a good job.I'll bet she takes it.

DEAR BOB: You recently stated that 60 percent of long-term capital gain profits, such as from the sale of real property, escapes income tax. What happened to the 15 percent tax preference item tax? Vic S.

DEAR VIC: It was repealed by the 1978 Tax Act. While there is a new "alternative minimum tax" for rich taxpayers, it doesn't affect most property sellers. Consultation with your tax advisor will clarify the entire capital gain tax picture, but generally only 40 percent of the long-term capital gain is taxable. The other 60 percent escapes tax.

DEAR BOB: What do you think will happen to mortgage interest rates? I understand they are over 11 percent in some areas now. We've been waiting to buy a home but interest costs just keep going up. Ella R., Quantico, Va.

DEAR ELLA: Home mortgage interest rates probably won't come down soon. There are too many inflationary pressures pushing them up. Many mortgage lenders are short of funds now because people realize it is foolish to keep money in savings accounts that earn less than the inflation rate. Buy a home now before its cost and the mortgage interest rate go higher. With home values appreciating at least as fast as the inflation rate, how can you lose?

DEAR ROB: The only real estate agent we know is the one who sold us our home. We want to sell some commercial property and want to know if we should list it with her. I say yes, but my husband says no. Lanni M., Alexandria.

DEAR LANNI: It's usually best to list commercial property for sale with an agent who specializes in it. Similarly, a home shouldn't be listed for sale with a broker specializing in commercial property.

Contact your local board of realtors for names of members who are commercial property specialists. After interviewing several, and checking their client references, list with the one you feel has the best success record and who will do the best sales job for you.

DEAR BOB: Is it true that next year the longterm capital gain holding period becomes 18 months? Howard E., Rockville.

DEAR HOWARD: No. There are no plans to change the current 12-month minimum.

DEAR BOB: Please explain more about why it is foolish to buy a house for all cash. We recently did so and, ince we are retired and pay little or no income tax, we feel it was smart to have no mortgage payments to worry about. A new mortgage would have cost us almost 11 percent and the best we could get on a savings certificate is under 10 percent. Hilda F., Arlington.

DEAR HILDA: You problem now is that you've tied up you cash in your home, you will have difficulty getting it out without selling. Even though you would have lost 1 percent interest or so by getting a mortgage, since you home is probably appreciating in value by 10 percent or more annually, the 1 percent differential is insignificant.

Advantages of getting a maximum mortgage when buying a house, or any property, include (1) repayment in cheaper, inflated dollars worth less than today, (2) income tax savings for the interest deduction, (3) easier resale with a larger mortgage for the buyer to take over, and (4) minimum cash investment and maximum leverage profit per dollar invested at resale time.

If you had obtained a maximum mortgage, you could have invested your cash in other real property for a double hedge against inflation. As retirees, inflation is your enemy. But if your own several good properties, they should appreciate in value at least as fast as inflation.

In today's inflationary economy, it's best to either mortgage to the hilt or, as you did, buy property free and clear of any mortgage. But by paying all cash you've limited your options - so hope and pray you never need the cash which you have tied up in your home.