I have been watching sales on ebay. £850 is pushing it I think. Then there is the commission and rates involved and the hassle of bots and fake bidders, its not for me personally. In answer to your question, I only realised today that it was store credit, the guy in the store told me if I come in with my iPhone tomorrow he can refund me and then I can repurchase with the credit. He was really great.
Not sure how that works - as he is refunding you, but then the system see's it as a returned device, and he is then selling you a used item (even though its going back to the first owner)....didn't think they could do this.
Personally I'll be placing the iPhone X on Gumtree - it will be easy to get more then £550. £750 should be easily do-able and its a quick and easy cash upfront sale. Note that I also have another year of AppleCare+ so I was factoring that into the price too, which naturally increases resale value.
Nevertheless appreciate its not for everybody and some like the quicker option. Just £550 seems like a kick in the balls.