Beware Fakes
Yes, popularity and high demand on genuine collectible items created situation on the market when along with original subjects circulates a big number of replicas, reproductions and fakes. Old medals, rare and hard to find badges-patches reproduced in Russia and on the West as well. And for new and not experienced collector often hard to find the difference between real subject and newly made replica. But the number of faked medals, badges-patches is relatively small comparing the number of faked cosmonaut autographs offered from online auctions, websites of new and not established dealers, who still easy can provide "certificate" from himself, but not to provide solid provenance.
I was often asked by new collectors I've met "Is that real autograph of Leonov, Nikolayev, Popovich?" Then I ask this person to look at photographs of myself with cosmonauts. It leaves no questions where from autographs came. Feedbacks of dealers at E-bay auction shows how responsible dealer in shipping of item, but still do not show authentity of items sold. Autographs of Gagarin, Komarov, Belyayev, Volkov, Levchenko, Tereshkova ,Dobrovolskiy, Patsayev are always high demand items for every collector. But sad fact is that 99% of them I've seen on E-bay are faked.
I am sure same problem exist with American astronaut autographs. Buy from an established, knowledgeable dealers you know on the market for 10-15 years at least. And if you decide to get a rare autograph check with graphic experts. With my many years experience on this field I had sometimes problems to tell well done fake Gagarin from real one, but graphic experts clearly shows by lines if it is real or not. Often Moscow dealers "mix" faked signatures on group photos with real existing signatures. Often in group of real flown covers and real signed items noticed "added" surprises too. Items flown in space also high demand category. Thousands of materials claimed to be flown offered these days and it is wise to be cautious with "flown" covers, hardware etc. If you are after such serious collectibles and ready to spend reasonable $$ best recommendation is to contact cosmonauts directly.