Well this is an interesting topic. I began collecting collecting autographs for about 14 years ago. I haven't added very many things to my collection this year. It takes time to shop auctions and sites. It takes even more time to do "through the mails." I just haven't had the time lately.
You can certainly get authentic autographs through the mail. But you can certainly waste your time and stamps getting fake autographs. Celebrities sign through the mail more often than a lot of people think. The vast majority have their time in the sun that might last for years or even decades. But almost all of them reach a point if they live long enough where all of the assistants are gone, and they spend much of their days just like the rest of us. If they've managed to save a lot of money, they might even have more time on their hand than most of us would.
A lot of these celebrities genuinely like hearing from their fans, and they do spend a portion of their time corresponding with their fans.
Take the Three Stooges for example. There were six Stooges (seven if you want to count Emil Sitka). They did thousands of personal appearances where signing autographs was routine. They generally signed when approached in public. I'm not sure if they signed through the mail when they were on the road and doing shorts and later in their careers, movies. However I am sure that Emil Sitka, Joe Besser, Joe DeRita, Moe Howard and Larry Fine not only signed autographs, they wrote lengthy letters to their fans in retirement. When they were written in the 70's and 80's not very much of the general public thought much of the Three Stooges, so those letters were practically worthless. Today those letters, especially Moe's and Larry's, sell in the hundreds. One with good content might sell in the thousands. You couldn't have given Moe's canceled bank checks away in the 70's. Now they are worth several hundred dollars each. The assistants, if they ever had any, were long gone when they wrote those letters. (If they had time to write letters, wouldn't they have had time to sign a photo or index card?). And the Three Stooges were arguable the most successful comedy act in history.
Clayton Moore, the Lone Ranger, is another one. I have no idea whether he signed authentically through the mail when he was doing TV. I do know for a fact that he not only signed autographs through the mail in retirement, he wrote lengthy letters when asked a question. I doubt he had more to do the last twenty years of his life than my grandmother did. The assistants were long gone.
Kenny Stabler was one of the top football quarterbacks of the 70's (maybe top two or three). He wears a super bowl ring and took Alabama to a national championship. For the last twenty years he has never to my knowledge not signed an autograph when approached in public, nor has he not returned an autograph through the mail. He was one of my neighbors for ten years on the Redneck Riviera.
I could go on an on, but the fact is that celebrities do sign through the mail. They also send preprints, rubber stamps, and secretarials through the mail. Usually most of those go out when they are at the top. Tom Cruise and Madonna aren't going to sign anything for you. Nor would Marilyn Monroe and Humphrey Bogart when they were on top. When the assistants are long gone, you are more likely to have your autograph request ignored than to receive a fake (though it does happen). You stand a very good chance of getting a legit one.
But when you buy baseball cards, you could buy counterfeit, trimmed, or over-graded cards. Even the experts get fooled.
I collect arrowheads. Knappers are good at faking them. In fact, fakes sometimes end up in museums. People look at them and ooh and awe at the beauty and the history ... but they are fake. Some people even know how to fake patina. Some people like to knapp arrowheads, bury them in the ground them for awhile, dig them up and "plant" them in fields for people to find them.
Point is every hobby can have fakes, and often the more "fake-proof" a hobby appears on the surface, the more easy it is to get away with the fraud.
Of course meeting a famous person and getting an autograph is fun and those autographs are probably going to have special meaning to you, but getting back to the "you only know its real if you got it in person," argument, for me that wouldn't be a worthwhile pursuit. What good is a collection if you are the only one that gives it validity? When you die, nobody would want it.
I had rather have a collection that passes my own judgment. After awhile a collector develops an intuition about what is legit and what isn't. He learns to collect what others would want and would deem genuine.
I like to buy signed letters, documents, bank checks, contracts, autographed books from recent book signings, signed & numbered things. I have hundreds of those things in my collection. I also have many signatures and signed photos in my collection because I have decided that they are legit -- and I like them. .
Here is my new website if anybody is interested.
http://www.collecting-celebrity-autographs.com/