I believe there is a way to block individuals on ebay, but he'd probably go around it by using a fake acct.
I almost always try to think of online purchases as a calculated gamble, knowing if I don't know the person, I may lose money. Doesn't make it right, but yeah.
I bought some fish from someone on aquabid recently that came with some fry as well. Took time (as I'm sure the seller knew), but 3 of those fry are not those fish I ordered methinks. They are beginning to look like like black bar endlers getting their colors.
Naturally I had already left positive feedback from before, and the seller through e-mail denies he did it, I'm almost posative that's what 3 of those fry are. I know the gamble though, and I also know if I ever leave negative feedback for anyone, they can retaliate. The worst I do is leave no feedback at all.