When you go in to talk with the LFS further about buying the tank, or to get your $50 back, whichever you decide....ask about the leak again. Then ask them how they'd go about fixing the leak. Then ask them for more detail about the stripping the seals, or whatever it is that needs to be done (I have no clue). But having them say all this and talk about it for 2-3 minutes is going to make it IMPOSSIBLE for them to look you in the eye and say "$500 firm". After having spent 3 minutes saying what all is involved in fixing seals on a tank, how much time it will take to do just one seal (ask them that), how long it will take to dry, how it may require a re-seal, etc., they will take $400.....or $300. Get them to tell you themselves that there's work on your part, and THEN hit em with your counter-offer....