I've bought three 2nd hand 150-200L 3ft tanks for $50, $50 and $30 each, although one had lots of scratches from a turtle and the other two needed to be cleaned and re-siliconed. In saying this, these tanks were a steal. 8)
The trouble with second hand tanks is that there is lots of uncertainty as to the age of the glass, the condition of the glass, the condition of the silicone, the bracing, the glass thickness etc. If you give as much information as possible you should be able to sell it for a better price :thup: