i agree with craigslist. it usually takes a while, and you usually have to get it hydrotested.
in the end, for a 5# tank though, it isn't going to save you much.
on craigslist, most likely you'll have to pay around 20 for the tank, and then another 20 for the test, then another 10 or so for the fill. that's 50 already.
in austin, i can get a full, used, 5# tank for 65. when i heard that, i was amazed, i thought it was cheap... to me, that conveniece is worth 15 dollars. whether or not it is worth 20 is up to you.
with taht said, i didn't buy the 5# tank, as i currently have 6 co2 tanks i bought off craigslist. a 50#, 3x20#, 10#, 2.5#. most of these were worth the wait. the most i paid for any of the tanks was 20 dollars, the fills cost at most 15, and the hydrotest cost 18. so for 50 a piece, i had a full tank in test, which is a fantastic deal for a 20#, let alone an even better deal for a 50# tank. for a 5# tank or less, that deal becomes less of an amazing deal.
try to get the largest tank you can though. it makes co2 that much cheaper. seriously. the swap cost for my 50# tank was 13.50. for the 2.5# tank, it was 8. you do the math. heh.