can also look on local second hand sites like craigslist, offerup, and letgo.
mercari is another one.
Offerup, letgo, and mercari can be used on the site, but mobile app is best for notifications and such.
otherwise lastly, there's eBay.
I would highly suggest craigslist, offerup, and letgo as alternatives first since those can be done local where you could even see the CPU working in action before handing over cash.
Mercari is more like ebay and does shipping.
Not sure about letgo, but offerup allows users to put their stuff up for sale with shipping so there are a couple of options you could find that way too.