That R9 is only about as good as GTX1060:
https://gpu.userbenchmark.com/Compare/Nvidia-GTX-1060-6GB-vs-AMD-R9-390X/3639vs3497
And eww... only 128 SSD. Doesn't matter if it is two of them. Even new nowdays they are $20 bucks a piece at best, see:
https://www.microcenter.com/product/485877/120gb-3d-tlc-nand-sata-iii-6gb-s-25-internal-solid-state-drive-(120g)
Nzxt fans are crap. Case is slightly better than crap. And the 2400 ram is just average crap. Water cooling is just another thing to break, spring leak somewhere, not worth even $1 in my books especially used. HDD after 3 years is worth $0. You might was take it out and keep it yourself for backup storage if you still trust it. So ball park factor into risk of things not being really as good as the "good condition", as skeptical and suspicious buyer, I'd risk about $350 tops for it.
Seriously, with all new components, people can get a R5 1600 for $100, and a RX580 for $180 see:
https://www.microcenter.com/product/479525/red-dragon-radeon-rx-580-dual-fan-8gb-gddr5-pcie-video-card
https://www.microcenter.com/product/478826/ryzen-5-1600-32ghz-6-core-am4-boxed-processor-with-wraith-spire-cooler
... and fill out the rest and be ball park around $550-$650 for a system that is just as good with brand new, with warranty on every part. There has to substantial saving to go with the used route.
You'd be advised to part out the system, sell the GPU, motherboard, CPU, memory separately. You'll probably get more for it. You can then reuse your case, psu, water cooling in your upgrade/replacement build.