Short back or not, if it's lordosis, then it will get worse as he ages and there's no stopping it.
Also, the chance that you'll be able to sell him on if he doesn't work out for you is virtually nil. I mean, just look at the seller he's with now. She's had to drop his price to less than what the killer pays just to get anyone to look at him.
If I was to buy a horse like him (though I probably wouldn't actually buy him, instead I'd offer to give him a good home for free), it would be with the intention of giving him a forever home, regardless of how long he's rideable. If the owner was going to insist on getting some money for him, I'd have a vet check done to see if the vet thought it was lordosis or a simple conformational fault. Then, I'd find a trainer that would train him and teach me how to drive a horse for when/if the time comes that he can no longer be ridden.
Don't take this as being rude because that's not how I intend it, but IMHO, to look at buying a horse with such a serious fault and having in the back of your mind "Well, if he can't carry me or if he doesn't stay rideable, I'll just pawn him off on someone else" is very selfish. Horses like him are usually destined to pass from owner to owner to owner to owner until someone just decides that they don't care enough to find him a good home and he ends up at auction...where nobody is going to buy a sway-backed horse, regardless of how young he is. There, the only buyer is the meat man for a horse like him.