Problem is, it's easy to sell on ebay than anywhere else. I posted 3 S5's and sold them all the same day on ebay. I could put them out in the forums here and not get any real interest for weeks since nobody wants them anymore.
This. And you can get high prices on ebay that nobody on this forum would ever pay. I sold some miners recently and I was a little nervous when a person with only 16 ratings bid on it but they sold to people with more than 30 ratings.
Are you allowed to put in the listing that you will not sell to anyone with less than 10 ratings (or whatever cutoff)? In the buyer requirements you can only select to block users who "Have a feedback score of -1 or lower" which basically means they already misbehaved in at least one transaction.
Also it seems to me that if they gave positive feedback they should be essentially giving up any right to bitch about the sale later.