I have accumulated about 10 tanks over the past 2 years and the first 2 were used. I got lucky they were part of a package of scuba gear, and they passed hydro.
Then I set out to get the whole family tanks and what I discovered is that it usually isn't worth it to buy a used tank, unless of course you have a friend or friend of a friend and you get a good deal.
Even though the new tank may cost more, since it will hold its value well because of a much newer born on date, the cost of ownership is about the same, you don't really save any money going used.
Leisure Pro usually has a smoking deal on the XS Scuba tanks, they come with pro valves and it tells you whether the tank is on or off with blue and green rings by the handle. They also come standard with the nice hard plastic flat boots. I don't think you can beat the cost of ownership on these tanks, they will sell easily too if you want to get rid of them.