Depending on the tank 180/240 have the same foot print 6' x 2' so you are only gaining height with 220 0r 240. Think about the work involved in breaking down and pretty much starting over. I'n my opinion it's not worth it.
You say you want to trade tanks plus cash? Not that many people are looking to down size from 220/240 to just 180. People who sell large tanks either go up in size or they are moving to a place where they can't fit the big tank. I got my used 240 for $500 so If I was you I would buy the larger tank then sell my smaller one (or sell yours first).
You say you want to trade tanks plus cash? Not that many people are looking to down size from 220/240 to just 180. People who sell large tanks either go up in size or they are moving to a place where they can't fit the big tank. I got my used 240 for $500 so If I was you I would buy the larger tank then sell my smaller one (or sell yours first).