this is why
eBay has reviews and ratings systems in place ... I have bought several unlocked phones on eBay - for myself as well as family.
I never deal with anybody with less than 1000 transactions, and at least a 98% positive rating ... and I have never been screwed over ...
Reading a cross-section of the comments left by others (positive as well as negative), will pretty much tell you how your experience with that seller will go.