Wow that really sucks!!! I hate hearing about things like this. You've given this guy waaaaay more time than I would've. Also, you make a good point, if you can stock a tank this large then $500 should be a piece of cake. PAY UP 1salt!!!

Do you have any kind of documentation from the sale (PMs may even be good enough)? Maybe something you could use in court? I would pursue legal action if I were you.