I have 2 mini Mac which aren’t being used any longer

That would depend on the exact model year and functional status of each Mac.

See here to get an estimate: link-> Apple Trade In - Apple

The trade in calculator is half way down the page.

I would be interested in your Mac mini if it is 2018 or later.

I would be interested in another 2018 Mac Mini as well.

Problem is, that we can not use the Apple Support Communities for selling and purchasing used products.

see section B. Submissions 6. No advertising at > Apple Support Communities Use Agreement - Apple Community

Mac mini.