I know there are several websites which offer to pay you for your old Mac - eg
macbbuyback.co.uk
macback.co.uk
cash4mac.oc.uk
cex.co.uk
musicmagpie.co.uk
etc
But I don't know which of the many sites are reputable, offer a good price, and also make the process easy by sending you a cardboard...
I bought a new iMac (2017 model) like 3 weeks before the new ones came out. It is a great machine, and I don't really regret buying it, since it's almost the same to the new one, BUT, I want to sell it or trade it for a Macbook pro from 2018, since I need the portability.
I would be selling it...