Will Apple buy back my iMac?

Trading for an Apple Gift Card or for credit for a purchase is the only option Apple offers. See: Apple Trade In - Apple

If you don't want to sell it yourself, there are 3rd party options. One that I have used and was happy with is https://www.macmeanoffer.com/

Depending on the iMac, it may have some trade in value and you can check here:

Apple Trade In - Apple

That is the only option provided by Apple, other than the free recycle program if it does not have a value and you are unable to sell it through a private transaction.

If you decide to sell it, see: What to do before you sell, give away, trade in, or recycle your Mac - Apple Support