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Sounds a bit high, but tank prices vary from region to region. You could probably get a brand new 55 setup for around that price or just a little more.
If it's all in good working order, then it sounds like a good deal. Offer them 250$ and if they don't bite, than pay full price. Must people will deal with you a bit.
In the uk we would pay £300 for a decent tank let alone filters, heaters and stand etc. A 55 here would set you back £3-400 for a shop brought tank. My 95litre cost £100, so I'd take a deal like that over here if there were any.
On that particular setup if I were shopping for one in my area I wouldn't pay more than about $150 for it tops, and preferably $100. There's definitely no way I'd pay close to retail for a used tank though.
I wouldn't factor in the price of the fish or decor, unless you were planning to keep them.