value of old iMac

We can be of almost no help here with used pricing. These are international English-language forums and values vary widely depending on where you are in the world. A 2012 iMac that is today worth very little in the US might be expensive in an emerging society with limited tech resources.

I see significant differences in selling price between our town with no Apple reseller and a one 40 miles away that has a university whose book store sells Apple gear

As no one here wants to know where you are, we cannot can quote values. Your best bet is to look at local and regional classified sites like Craigslist that let you focus geographically.

It's going to be $0 most places...

Apple Trade In - Apple

https://www.mac2sell.net

A local shop might be willing to remove the hard drive from the iMac for you at no charge if you leave the rest of the iMac there, which they might be able to use for parts.