I've sold a bunch of stuff on Swappa (to me, better than Ebay for Apple devices - both for buyers and sellers). I've definitely noticed that certain products (Eg. laptops) have huge differences in Apple trade-in value vs "Fair Market Value" on Swappa, whereas others (eg. iPhones) have smaller differences.
I was originally going to keep my 128gb iPhone 13 Pro for another year. I had just gotten a refurbed version via AppleCare+ in Sep after my kids dropped my phone and cracked the camera lenses. So I basically had a like-new iPhone 13 Pro w/ new battery. But as holidays approached, I decided I wanted to get the iPhone 15 Pro Max in Dec and got it a month ago.
Apple was offering $480 for my iPhone 13 Pro. Swappa showed for 128gb, sales for "mint" iPhones were around $525 - $550. Maybe I could've gotten a bit more given I had 100% battery health. But even Swappa (better fees than Ebay) takes 3% fee + 3.5% PayPayl fee + Seller pays shipping (~$20). Even if I could've gotten $550, net I would've gotten $495. Vs the super easy trade-in at Apple of $480, just made sense to trade-in and not go through the hassle and potential risks of selling online. I see iPhone 13 Pro trade-in value now now dropped to $420.
Just worked out that way that with my particular phone at that point in time. Doesn't always.