I would leave them negative feedbacks, they bid 
and when contacted should pay (only could see 
some dire emergency circumstances that I might 
forgive someone for backing out).  If they leave 
a negative on you, just post a civil reply to it 
that basically says that it was in retaliation 
for your negging them due to not paying.  

I know if I see neg feedback in a sellers profile 
I try to find the recent ones to read what was 
said.  A lot of time they are from brats who 
didn't pay and then got mad about a negative, so 
I ignore that type and bid away.