Did you forget to include the motherboard in the estimate?
Yes
What kind of fans?
Otherwise, seems pretty reasonable to me
Cooler Master 120s, RGB but not enabled.
Yeah, I think your overall assessment is probably good. Fans might be a little more, but they're not $30 Noctuas. I probably wouldn't go over $600.
That would put the GPU cost at about half the cost of the rest of the machine, which seems pretty reasonable to me. You can get storage for cheap at Christmas.
I'd say $675 if you add $100 for the motherboard.
$120 listed on Amazon
Used I expect a huge discount. Previous owner could have mistreated the hardware and there is no warranty. $300.
If you're trying to sell it to geeks, build it up in PC part picker, send the link out with the advertisement, and then sell it at 40%, maybe 30% of the new value.
If you're willing to do hand holding, you can sell it as an entire system, to somebody technically non-savvy. Higher margin, more work
If you're lucky you may be able to sell it for $575 but the more realistic price is around $500
Buildapc