Glasscages.com seems to have cheapest prices.
Also, if you think about it:
you have no idea what has gone on with the tank
no idea how old it really is
no idea if it's been repaired
no idea what condition it REALLY is in
if you buy new:
it's going to be freshest
it's going to be backed by a warranty
you know what condition it's in (new)
/shrug just my thoughts on it after I was lookin at CL
for a larger tank...
I had to think about the possibility of stuff going bad sooner with a used tank than a new tank since I live on the 6th floor :|