99% of the autographs on eBay are fakes and I'm shocked they don't do more to police their site.
Part of my job working in music has been getting various posters and CD's signed for charity events and contest giveaways. I've given a couple Pinsiders I'm friendly with, pretty cool pieces of signed memorabilia that I know they appreciate. One day I'm going to start selling down the hundreds of signed posters, albums, CD's, etc that I have. Anytime I did a signing, I'd always keep a few signed items for my own collection or to give to people I know are fans of the artist. Half of them are from up and coming "hot artists", who's careers never took off -so lots of junk. My old boss who died last year of cancer recently had a massive collection consigned on backstageauctions.com. That's a great site to get authentic items from people in the business. You'll pay a lot, but the stuff is all legit.
The most expensive autograph I ever sold was an old record company promo B&W 8x10" of Metallica with Cliff Burton. I found it tucked away in a file cabinet from a coworker who didn't' take it with when they left the company. I remember that sold for $800ish on eBay about 10 years ago. There were two and I still have one. It's one of my prized possessions. I also have an 8x10" record company B&W promo photo from Geffen, that has the signatures of the original Guns N Roses lineup. That's another one I'll never sell.