It's worth what someone would pay for it. Used PC's don't get a lot of money. If it's a gaming rig, gamers rather build their own, and a non-gamer won't appreciate or pay for a used gaming rig and will go buy a PC from Walmart that's new.
List it online, and see what you offers you get, or set a price or best offer and see what people offer you. You probably won't get anywhere near what you think it's worth though. Old intel chipset, few generations old CPU, GPU that won't most modern games at ultra 1080p anymore, a no-name crappy PSU, and a HD that has years of read/write abuse on it.
I'd keep it as a media server PC for a TV, or turn it into a file server, cause you probably paid a good chunk for it, and won't get that out of it.