While I would prefer to see Macs get repurpose by selling them to someone that would use them, or even giving them away to someone in need, I think Apple's trade-in offer for your Late 2013 iMac isn't bad, and logically, I would probably consider doing that instead of trying to sell.
Here is my reasoning:
I am going to try and sell my iMac before sending it in to Apple for trade in.
IIRC, Apple's Mac trade-in values tend to work out in favor for the base models, as I don't think they take in account BTO options and such. Either that, or they do not increase the trade-in value as much. I could be wrong about that, maybe someone else could chime in about Mac trade-in values.
If I am correct about the trade in values, and since you have the base model, then what Apple is offering you might not be too far off how much you could get otherwise.
For people that have a bunch of BTO options, I don't think Apple's trade-in value would be worth it.
You could make more trying to sell your iMac, but because of the Apple trade-in values benefiting the base models, trading in your iMac to Apple might be worth it instead of dealing with the risks of other methods.
It totally depends on how you plan on selling it and how quickly you would like to make a sale. I like to use ebay as a general guide.
Are you in the US?
On ebay, your model and specs are going for $300 with free shipping for buy it now, so this is usually higher than what an auction would yield.
You might be able to get a little more if you have the original box, but since it is a common model, I would use $300 as the high end if you get lucky.
While ebay would probably give you the highest selling price, there are fees involved with selling on ebay and the risk shifts toward the seller compared to other methods.
If you sell it on the MR marketplace, I would think that the amount you would expect would be about $150-$200, plus shipping costs. You might be able to get more, but it may take a long time to sell. Places like the MR marketplace, the risk shifts to the buyer, so they expect a good deal to make up for the increased risk.
I would expect a similar sell price for posting locally, like craigslist and such.
Of course, I could be totally wrong, and you could get way more than what I posted.
Another factor that might influence the selling price is the M1 Macs.
The M1 Mac Mini is a beast, and the cheapest new Mac that Apple sells. One can find them for a little over $600 on sale and/or discounts. It runs circles around the base model Late 2013 iMac.
Compared to many other electronics, Macs typically hold their value for a long time, but with the entry level Mac packing the punch that it does now with the M1, it could really bring down the price of older Macs, especially base model and models that are not typically for collecting.
