I guess it depends on what they have really done. If they are blistered I wouldn't buy them. I'm no tyre expert and not very experienced with this sort of thing but I would be looking for a pair that had no signs of cold tear (my supercorsa's were ripped to bits when I replaced them, Steve at Wraysbury Tyres had a good giggle when he was taking them off!) and just general wear.
Any deformity's like blisters etc would steer me clear of them. I'd try and find out if they'd been used with warmers etc as well to find out how many heat cycles they'd been through.
It would be a bit iffy buying them through e-bay but I guess you could get some bargains. Good luck, let me know how you get on because I might give that a go myself
TP