If you think about it, the guy selling a single used amp usually has little to lose if he misleads you, since he can quickly change his online identity and do the same thing if he needs to later. It's the online version of the guys selling stuff in vans. But the person or company that's doing repetitive business has a reputation to protect. They may charge a little bit more, but they've also had to develop their business. There's not really any comparison between these two kinds of deals.

I'd say the safest thing is to be prepared to lose anything purchased online from a person using an anonymous moniker. It's a roll of the dice. If it turns out well, super, you got a good deal. If not, it was maybe worth a shot but now you have a boat anchor.