I would hope they would at least give you half off, but thats probably not realistic.
You might not need to take it to the vet, especially if the eyes are still open. I had 4 turtles come down with a respiratory infection in early June. 3 of the 4 were not good about eating the fruits and veggies. These turtles who were in an outdoor pen got where their eyes were swollen, and their noses were bubbly, at least a couple got where they weren't opening up their eyes. I put them in the house in a very large rubbermaid tub I have. I put a 50 watt bulb in a reflector over one side of the tub and I had a large reptile heat pad that covered maybe 1/3 of the tub, under the tub.
I kept two water bowls in the tub and made sure each got at least two lengthy soaks in room temperature water, a day.
Then I started force feeding them baby food (gerbers chicken, with a few drops of cod liver oil(vitamin A) and reptile vitamins mixed in). At the beginning of treatment, I also soaked them in pedialite and mixed a little pedialite in with the chicken.
Some people tube feed (get a feeding tube down the esophogus). I've never felt comfortable doing this (would like a vet to show me how sometime) so I gently pry their mouths open with a large paper clip and squirt some of the mixture in their mouths. In my experience that they almost always swallow it, although I make sure I give them plenty of time between bites.
All 4 turtles fully recovered within 2-3 weeks and all have improved somewhat in the eating of fruits and vegatables.
This turtle you are talking about doesn't seem to be as bad as mine were, but if his conditions don't improve, he will get worse. We have a pet store in OKC that I used to consider pretty good, the only animals I've seen that I thought were seriously mistreated were box turtles, and I had never seen them have box turtles until fairly recently. I don't know why pet stores in general seem to be so careless with box turtles. In all cases I've seen, they didn't have adequate water.
Well keep us posted. The others make good points about not buying him, but you've got to do what you feel is right.
Maybe, if nothing else, you can get them to take better care of him.