Ebay's fees are out of control. It's good if you just want to get rid of something but holy crap they rob you.
But selling locally is a pain and a hassle too.
It seems Apple is about the only trade in place that you can get a fair offer with no hassle.
With Ebay in particular, it's not so much the fees for me (which I agree, are high, but you also get the most eyeballs on it as the largest secondary market place), but all the scam buyers and how "buyer friendly" Ebay has become. They essentially side with the buyer on almost any dispute - and over time, there's evolved to be a lot of scam buyers out there who, eg. 1) claim the product isn't as described ask for a refund / discount, and/or send back a different device, or 2) claim they never received it or that you sent something completely different (and just swap in something else). The number of stories I've heard on what Ebay has become, I know a lot of ppl who used to sell a ton, but after getting burned on one too many transactions where even with decent proof, Ebay sided with the buyer, and no longer sell on the platform... Or you try to do on FB Marketplace, and you get robbed...
Trying to get 5 or 10% more selling on Ebay (even netting out Ebay fees) vs Apple Trade-in, will be completely dwarfed by even a single scam buyer who hoses you out of an expensive piece of Apple tech.
At a certain point, it is worth it to try secondary market if the Apple Trade-in price is that much lower... but I find people often "misprice the risk" of getting scammed out there today. So you just need to appropriately factor that in when you compare the Apple trade-in price.
The other thing that's a decision factor with iPhones and iPads in particular is whether you have the base model vs upgraded storage. Apple trade-in prices for iPhones and iPAds are the same whether you have the base storage or an upgraded storage tier (I think with Macs, they do take it into account somewhat, at least on Memory). So if you're someone who paid up for 512gb or 1TB+, all things equal, the Apple Trade-in makes less sense, and you may be better off trying to sell privately...