Power supply Value?

Disclaimer:I don't know if this is the proper Subreddit to post this on.

So recently I found an old PC in the street w/ a SIngle core Celeron,1 gig of ram,some 40 gig hard drive,and a HD 7500,It didn't work (No shit),but heres the kicker It had a 500W supply,it doesn't have any brand stickers or efficency rating,but it's 500W,I've been wanting to sell it,but I don't know it's value,and I don't want to either under value it or over value it,

thanks for contributing w/ your opinion,have a good one.