Here's what I know:
Hardware(including S3) have a 1 year warranty(so a used S3 may be out of warranty already)
For the warranty to remain effective for the full year, the seller would need to; A-have the S3 registered. And B-provide a transfer of ownership document(providid by Avid).
Having said that, my best recommendation is to look for a great deal and not to expect any warranty at all.