The CPU and GPU are quite dated at this point and computers do not hold their value. Never have and never will. That monitor is also a bit small. Now I think 250 is probably low balling it a bit but I also do not see you being able to get "at least" 500 from that system either.
CM HAF 912 $60 | FX 6300 $90 | Hyper 212 Evo $31 | Asus M5A78L-M/USB3 $50 | Corsair Vengeance Pro Series 8GB (2 x 4GB) DDR3 DRAM 2400MHz $68 | Corsair CX600 $67 | AMD RX470 $170 | WD Blue 1TB $50 | Sceptre E225W E225W-1920 22.0" $80
So there I just built a system off of Amazon parts (new not used) that equals or exceeds the performance of your system for $666. You might want to re-consider your expectations of that system's worth.
LOL nevermind Chugalug beat me by a country mile.