I have 2 tanks that I want to sell but I have no idea how much to sell them for. I have a 20# and a 15# C02 tank. They are older but very heavy duty and in very good shape. What do you think I can get for each of these. BTW, I am guessing that is the size. The smaller one of the two has 22 1/2 lbs stamped on the neck. The bigger one is almost the size of a corny keg.