Three things:
1) Are we talking US dollars? If so yeah that $400-$480 R9 390 claim is bunk.
2) Confirm it's a valid licensed Win10 copy.
3) Ask him for FULL build specs including motherboard, memory, CPU, case, and CPU cooler make and models. Those can validate or invalidate the price he's asking with your $600 counter offer which no matter what he had, I'd not pay more than that for (again, if we are talking US dollars here).
BTW: used R9 390s are going for anywhere from $250 to $350 right now on eBay (US), so he is inflating that value a little.