Not much money can be made from selling used tank, unless you can get them for free or in a very cheap. Also, the market don't have a constant supply of used tanks. Sometimes you will find a few, or sometimes you won't find any at all...
Here are some photo of my used tank sales... (size ranging from 25G to 90G tanks)
Even now, I have a few tanks that I wanted to sell
55G with wooden stand / light / canopy asking $180
75G with wooden stand / canopy asking $250
35G asking $35
AND believe me! I got them cheaper than what I asked for
At one point I have 50 / 40G tanks in my basement / garage, enough to open my own fish store