400 seems pretty low, for us on the forum who know what parts are worth though it seems about right, but the general public usually overpays. I just don't know by how much.
It's a used entry level computer (albeit with what was a mid range gpu) from several years ago (not sure when you bought it, but the hardware is old tech). I think you would be ripping off whoever buys this if you sell it for more than $400.
All those parts new together cost around $400 so considering you can buy much better newer equivalent parts for that and the computer is used i would say $200-250.