Looks reasonable, starting in gear is normal for a Wrangler. You are expected to depress the clutch but when submerged you start in gear with the clutch engaged, something to do with water affecting engagement.
Looks pretty clean. $5700 sounds like top dollar but that is the beauty of Wranglers, they hold value better than anything.
As mentioned, crawl under it and check the frame. Tapping with a ball peen hammer, it should sound the same, front to back.
The tailgate might just need to be adjusted/lifted since worn hinges let it drop.
He fixed the transfer case himself ?????? Frightening, get a proper mechanic to inspect it.
122K miles is nothing, sounds like a TJ which was parked more than it was driven. If it was always parked indoors big plus which explains why it looks so good. An inspection will be the best $100 you ever spent.
How many TJs does he have? Is he just flipping them ? Almost looks like a used car lot. Verify everything he says. This one "looks" very clean, something curbers are real good at.
All stock, no mods, low mileage, clean interior, original seats with no wear, plus, plus, plus. Just verify the mechanical and rust.