Will you be going to see the horse? If so, I would take your own camera and a partner who is familiar with setting up a horse for confo so that you can take your own pictures. If not, I would send the seller a request for additional photos and specify that you would like unsaddled, squared up confo shots from the appropriate angles. A good seller should be happy to oblige (well, a good seller would have already provided them, imo)
Okay, not sway-backed. But those first two pics definitely make him look it! Lol
I still see a very long back and prominent withers (now that I've seen him without a saddle...which would explain the cutback saddle pad). Really, other than that, I like him...and even those "issues" are manageable.
The shape of his bak makes me worry it will sway as he ages, it's a hair long with a prominent withers and..... I don't kno how else to describe it but "jumper bump" croup. His hind end also looks a bit under developed, I'd like to see more muscle. It also looks like he needs more heel on his back feet, and if he can't grow it, it might eventually require wedges. I can't really see his angle on his front feet. Being picky: he could use more topline :p
Personally, I really like him. He reminds me a lot of an Appendix that I work with that we all adore. I see the long back and high withers, but I have a 17 year old QH that's the same way that's always been sound. Keep us updated if you get new pictures of him :)