Leaving negative FB for buyers does no good. It does not stop them from continuing to buy or bid. If they use BIN or snipe bid at the end of an auction, there is no way to stop them, no matter what their FB says. There are millions of Ebay users, so it is not feasible to try & block everyone with bad FB. Most of them would not even be seen until after they purchase & then you are locked in to complete the sale, so what good is that?
The UPI system allows sellers to block non-payers with 2 or more strikes. If sellers have their blocks in place, these buyers can not buy or bid. So they are stopped.
The FB system is one-sided, so it makes it useless.
@darco: Sometimes sellers do get the shaft. Ebay gives you tools to use to help avoid this as much as possible. Put your blocks in place, ship only to the address in Paypal, make accurate descriptions & ship on time, pack carefully, use delivery confirmation, etc.
As far as a class action suit, I'm not sure what you would base it on. You would have to have your lawyer read all the fine print in Ebay & in the agreement you approved when you signed up.
When you join a site like this, you have to agree to their terms. You have to learn & know the rules. Many sellers make mistakes because they don't know the rules & policies, then when something happens, they complain or blame Ebay. One of the biggest problems is that sellers don't follow up on disputes or refund before they get their items back. So they end up losing the item & the money.