Tank deals can be found, but it's generally harder to assess tank condition online on used, and shipping costs make online deals on used tanks less so. I know some folks that have picked up used tanks locally for a very good price. In landlocked areas like where we both live, those deals are more difficult to find as a lot of locals don't own tanks.
I recently joined a Scuba club where the Shop provides free tank and weight rental, and free fills for a nominal annual cost. In this case $35 annually. That is one alternative assuming a shop in your area has such a thing.
I picked up a couple of used AL80's for $160 a while back. They had recent hydro's, and new VIP's. This was a FL dive shop that went out of business, and apparently the owner had family here, and drove a truck full of stuff to Denver. A bunch of my friend bought tanks from this dude as well, but it's kinda a rare occurrence.
Another problem with purchasing NEW online is that they must empty the tank to ship. A lot of LDS will charge the cost of a vis if the tank is empty adding $20~30 to the cost of a new tank. There is no reason they have to do this, but they don't like to see folks purchase online, and they punish those that do if they can.