SellGPU will offer $602 USD, today for any 2080TI.
When the next top tier card launches, that will reduce. Compare a 1080TI to give you an idea of how it might age (not exactly, but you get the idea)... Sell GPU offers $252
The absolute lowest a (working) 2080TI has sold for on eBay, ever (as far as I can tell) is $450... and that was an inadequately cooled reference style design..... and I'd probably question the validity of that sale.
The lowest recent 2080TI sale on eBay is $760, auction for a blower system...
Typically, a decent 2080TI will sell for $800-$1000 all day long.
Even accounting for eBay fees.... the low end $800 less 10% = $720 / 20% more cash in your pocket/PayPal.
To compare to the other cards, a 1080TI sells for ~$400 pretty easy.
So, beyond the convenience factor.... no, SellGPU is not going to net you anywhere close to a fair market value.