Reiterating what has been said above, older GPUs and semi-recent hardware in general is retaining value. You're probably better off just targeting some new hardware. A 3080 or a 4070 Super is probably what you'd be looking at for your GPU, and the cheapest I'd expect a 3080 to go for is around $700, which doesn't leave much room for the rest of your system in the budget.

I suggest saving up a little (lot) more and buying new if that's your target build, otherwise reign in your expectations a bit.