bidding in an auction seems like a binding contract to me. if someone bids and goes missing when he has to pay i think he is a scammer.

but no money is lost; so its hard to tell.

what about this scenario:
someone makes an auction
a few people bid
highest bidder goes mia for a week
seller sells to the second highest bid
highest bidder come back and wants his item... in this case the seller is the "scammer"

so please just write your auctions carefully and handle that case (eg highest bidder needs to react in 48h otherwise next one gets it)

in none of this cases i'd leave a negative rating though. but i'd place a neutral and i wouldnt trade with him.