I guess. Depends on the situation to some degree. I sold a Game Boy Advance + 3 games about a month after it launched. I recouped most of my money, and didn't have to wait long, selling to the first person who came to my place look at it. I went the trade route for the XB360 about a year after it launched. The console, 22 games, extra controllers, various other accessories......It could see it being a pain in the ass to sell privately, so I just took it into EB. Took store credit, which bumped up the $$$ value. Still took a hit on it vs selling privately, but well worth it to avoid the parade of strangers coming into my home, vs being done with it in 30 minutes. IMO.

I imagine PS4 Pro still sells better in most, if not all, markets, so it makes sense for it to have a higher trade in value. the retailer can flip a bigger profit with it, instead of having it collect dust and take up shelve/ inventory space.

Well the PS4 Pro seems to be more popular due to the quality exclusives it has being more appealing to the masses. I myself went with dynamic 4k experiences with the pro as I play more than just multiplat games.

Wow even looking at CEX prices the X is definitely lower per normal full price

PS4 has more demand, so it's trade in is more. The same goes for different makes and models of cars, and many other things. Supply and demand.

Should be simple, Sony just didn't deliver many PS4 PROs the last months so that Gamestop needs to pay more for used ones to get at least those. But Gamestop has enough new Xbox One X consoles because MS ships enough. That means Gamestop doesn't have to tell every customer “we don't have it new but you can buy a used one“

If Sony would ship twice as much and less people would need to buy a used console, Gamestop wouldn't have to try so hard to get used ones.

I think it's more to with the fact that there is somewhat of a shortage of pro's in the US, so Gamestop offers higher trade-in value for that. Makes sense.

Wondered what they are selling for used on Ebay: 

Cheapest XB1X: $359.90 (including shipping)
Cheapest PS4 Pro: $409.88 (including shipping)

Wow, PS4 Pro really is in high demand right now!  Might as well buy a new one.  (Or conversely, you can get a really good deal on a used XBox1X if you want.)

This. I myself am strongly considering getting a Pro and theyre not in stock anywhere and shipments seem to have stalled. I’m thinking I’ll try for one of those fugly Spider Man ones or, failing that, just get another regular PS4.

Gamestop Trade/Sell values are based on the traffic flow.

NEW PS4 Pros are out of stock at most retailers. Something is going on with them over at Sony where they have not been restocked in over a month. This means the demand of any PS4 Pro is higher right now, Gamestop really wants some of them Preowned so they can sell them. Not to mention PS4 has Spiderman and NBA 2k19 that are show titles for PS4 Pro right around the corner.

Meanwhile...

Xbox One X channels are stuffed, and have been since last holiday. They didn't start to really increase their sales numbers until the Gamestop's "Trade PS4/XB1/Switch console toward XB1X and get up to $300" deal.