There are two places that come immediately to mind:
The first is everymac.com, which has an article on every Mac that ever was. At the end of the article is an estimate of current street price. They are typically less than 6 months old.
The second is eBay. The technique is to do a search for what you want to sell. and keep restricting it until you get almost no outliers (stuff that accidentally ends up in the search, but is not applicable). Then you click on [√] Completed Items (recent sales) and sort by price. Then discard the lowest, and read the items and move up the list until you find one sold as a good system, not parts, not "as-is", and was an actual auction with multiple bidders. That is what you could buy one like that for on a good day, or sell yours for on a Bad day.
Put some time and energy into making yours sound better, and you may convince some folks to bid higher. Buyers want "ready to go" stuff. Anything that will take some fiddling to make it go is risky, and will have a much smaller pool of potential buyers.
Note: what is discouraged here is the actual haggling over the price and arranging purchase/sale. Discussions of HOW and even estimates of value are not a problem here.