(My experience: I collect Harry Potter books. Signed copies were always faked, because it is "easy" to forge a signature. But lately I've been seen fake first printings. Some people buy 2nd or 3rd printings (way cheaper) and change the page with the bibliographical details, which is hard to spot. Other ones even printed the whole book, those are a bit easy to spot because a 25 years old book shouldn't look as new, but if you buy it online seeing only photos, you may get scammed after paying thousdans of dollars)