I just purchased a used PSP on eBay knowing that it had a few scratches.  The seller noted this is the listing.  I did not think it was worth the extra $50 for a new system so I purchased the scratched unit. (never again).   The system I purchased works 100% like new, but the scratches really bug me so I just purchased a scratch remover kit (hope it works).  By the time I get through the $50 I saved was not worth the bother.

The bottom line don't buy a used system that has scratches.  The only way I would ever consider a used system again is if the seller guarenteed it worked like new and looked like new (no dings scratches etc).