First of all, since you don't specify your location, it's difficult for anyone here to give a suggestion unless they tacitly assume you live in the lower 48 United States. There are plenty of international MR forum participants and Mac pricing (new and used) vary wildly.
The best way to determine a fair price for anything used (not just computers) is to look at similar items. For these types of items, eBay and Craigslist would be the goto online marketplaces for comparison here in the USA.
Look at those types of places to see what recent transactions were. The real estate industry has a term for this: "recent comps" (recent comparable sales).
Sites like eBay and Craigslist are infested with scammers so sellers really need to have their wits about. Any hint of suspicion is a big red flag.
Some people consider private party sales to be too much of a hassle and would rather sell it back to Apple for store credit. This never generates the same amount of money as a private party sale would. It's the same if you trade in your used car rather than sell yourself; the dealership never offers what you could get in a private party transaction.
You could plug in your system into Apple's buyback website and see what sort of quote it generates. The big negative is that you will be dealing with Apple's buyback partner Phobio who has a decidedly mixed track record. I've had good and bad transactions with them.
The last time I did this, I received a decent preliminary quote from the Apple buyback site, but upon physical inspection, Phobio knocked down the value to nearly nothing. I refused their unacceptable lowered price (the unit was pristine) and they returned the computer. A couple of months later, I decided to take it into a bricks-and-mortar Apple Store for an in-person assessment by an actual Apple employee. Their quote -- while less than the original online quote -- was acceptable enough for me to let it go.
