Around my area, decent working tanks seem to go for about 50-$75 with a hose and fitting. Not sure on non-working ones.
I don't think you can test the tank for pressure by putting gas in it and pumping. The pump is designed to fill the fuel hose and fuel bowl/carb with fuel. It will not pressurize the tank, at least on mine they don't. I think you would have to connect it to a motor and run it to pressurize the tank. Although, I have had tanks that leak through the pump when trying to pump them when not connected to a motor, but I'm not sure if thats an accurate test.
I have had tanks that will "hiss" several times when opening the cap after being pressurized. They can become pressurized from the sun or high temps just like a regular gas can also. I would say thats a good sign if they do that. I think I have seen a post on here about that jig but I have no idea what post it was in. And my only complaint about this forum is the "search" feature. Maybe I'm doing it wrong but I either get zero results or 8,422,653 ( or at least it seems like that many) results when I search. I think I also read with the right tool ( and not meaning an OMC tool, just the right pointy thing) the job is not too difficult. But i've never done it myself.
I would guess if you had gas in it and had the cap tight and everything was sealing tight, if it was in the sun it would build up pressure.
Very fair. I would ask more if both were working. It's so fair that if you deliver them to OHIO for me I will even give you a $10 tip.
Did you see that link on the o-ring replacement?