DEAR ABBY: I read about the Pasadena lady who got ripped off by a man she hired to repair her roof. He wanted $400 in advance to buy the tiles. She gave it to him, and that was the last she saw of him.

Speaking as a self-employed mason and repair specialist, I resented your saying. "Fly-by-night laborers who don't have the materials to work with are obviously not professional." There are times when I ask for money in advance to buy materials because I can't afford to have a large inventory sitting around.Of course, people who buy services should always check out the craftsman first. (I always have five or six references.) But please don't label all laborers who ask for money for materials in advance as "unprofessional." I am a pro and I've never ripped off anyone in my entire career.--JOHN E. DENISON, DALLAS

DEAR MR. DENISON: You aren't the only repair specialist who complained about my answer. But be fair, John, the man who ripped off the lady in Pasadena was a stranger who rang her doorbell and offered to repair her roof. He offered no references, and she asked for none. (Big mistake.) I said that he was an irresponsible, fly-by-night crook--which he was--but I didn't mean to tar all roofers with the same brush.

In some instances, the craftsman is ripped off by a little old lady who does not come up with the money after the work has been done, so I recommend that a contract (or agreement) be drawn up in advance to protect both parties.

DEAR ABBY: A couple of months ago, I received a letter from a man I had never heard of. He says he was laid off work and flat broke and bill collectors were hounding him. But in January '84, he and his family took a 10-day cruise, he was able to buy a brand-new Cadillac for cash, and all he had to do was write some letters and send a few dollars to people whose names he got from a mailing list. It sounds so easy, and he says it is 100 percent legitimate.

He has written to me three times. There is no return address on his envelopes, so I can't write and tell him what I think of this, but I know there has got to be a catch in it somewhere, so I am not following his instructions and sending a dollar to the five names listed.

I am enclosing his letter and all the instructions so you can see for yourself. What do you think of this? I can't believe a person can make $50,000 in four months doing nothing. He says it's a legitimate business opportunity.--BERTHA C. SHELBY, OHIO

DEAR BERTHA: This get-rich scheme is a scam. There is no Santa Claus, tooth fairy or free lunch. Chain letters requesting money are illegal. I am sending this "Business opportunity" offer to the postmaster, which is what every person should do when a letter of this kind shows up in the mail.