Warranty vs. "authorized dealers"...
There's nothing in the warranty terms, as listed in the PSR-500 manual, that says the warranty is only "valid" if you purchase the PSR-500 from some "authorized dealer".
As long as you get an "original purchase receipt" dated within the last 12 months, GRE should/will honor the warranty. (I presume that "original purchase receipt" would have to be either a) from GRE or b) from someone to whom GRE sold the scanner.)
So, if someone purchased 20 PSR-500s from Scanner Master and got 20 "original purchase receipts" from Scanner Master, then sold those 20 scanners on eBay, the end purchaser should be good to go if he gets one of those 20 receipts from the eBay seller.
This would be no different from me buying a PSR-500 from GRE America and giving it to a friend as a gift (along with the GRE America receipt), then having the friend contact GRE America for warranty service.