You may not be the first guy who caught him on that.
Chances are maybe 25% that this tank will crack when moving it, anyway, right where the chip is.
There may be chips under the trim too. They're never a good omen.
If you still want it, wait another week, or two and see if it's still for sale. Then low-ball him.
You will likely spend a min of 25 man hours to move this, disassemble, scrape, and reassemble the tank.
It's about how cheap you are willing to work for something that has no guarantee when you are done.