In general, the rule of thumb for used aquariums is $2 per gallon - so a 150 gallon should have a minimum price of $300. You do not mention a stand or hood, or accessories, such as lights, filter, etc. What is the condition of the glass tank? Any chips, cracks, or scratches? Flaws always lower the price. How old is the tank? Is the stand metal or wood? I would check the internet for prices of used tanks similar to yours, then compare and price the item accordingly.