Can I trade in my old iMac if I bought the new one from Best Buy?

You can trade in almost any Apple device. The trade-in value will be issued in the form of a gift card. Items with no trade-in value can still be sent in for recycling. Go to the bottom of most Apple pages (not ASC, though) & click on "Apple Trade In" & follow the steps.

There are other sites such as Make Me an Offer that will accept trade ins.

Yes, absolutely and +1 to Peggy's excellent advice.