I would not recommend buying one for $600, it probably either won’t arrive or will be broken. Someone will reply saying they got one for $600 and it works, that doesn’t mean it will happen if you do it.

I’d say the market is realistically $900-1100, maybe $800 if you know the person or can watch the card running first.

All that said, this advice will expire in a month or two when the 5090 comes out.