My current PC is 7 years old and I've actually already ordered new parts to build a gaming rig. I wanna price-check my current PC to see how much I can sell it for. I'm going to list very basic components.
You could get $300 or more out of that on ebay. You might be able to get $500 out of it but that's doubtful. Once I bought a GTX 470 graphics card on ebay for $100. Then I sold it for $160 a month later. Then after that I bought another one for $100 again. It seems like I could keep doing that over and over again.
It would vary by who you were talking to. You'd be hard pressed to get more than $100 from me for that system, but someone lacking in computer knowledge or with low requirements for their system might give you $200 or $300.