Make Sure you buy from someone with high approval ratings, not a newcomer.

Also ask for a C.o.d.

See about their return policy.

Study the add and make sure you are buying a real authentic autograph and not a preprint or a copy

Don't overbid unless you are sure you are able to pay for it.

Check the buyers other items to see if they are selling more than one of your "hand signed original"

Don't be surprised if you end up buying a fake....another reason you should not spend too much on ebay!