Many sellers don't leave FB at all rather than leaving a "positive", even a "twisted positive". I know many sellers are not happy with the current ebay regime but it is the proverbial pendulum which is now swung in favour of the buyers. IMO, this is not totally a bad thing. I do think that buyers are more vulnerable than sellers and that there are more scammy sellers out there than scammy buyers.
IMO, the most common buyer offence is not paying. Annoying, yes. Scammy, no. If ebay would deal with these NPB's firmly and consistently, i.e. three strikes and you're out, that would be a good thing.
IMO, the most common seller offence is underdescribing the condition of an item. This goes beyond annoying and can verge on scammy if it is bad enough.
The buyers and sellers from hell that deiberately set out to scam someone or otherwise make the transaction utterly miserable are not that common.