If the savings wasn't too high ($20-30) I'd probably buy new. If you buy a used tank, who knows what stupid things the previous owner did with it. Carrying tanks even a little full can stress out the seams, and cause them to fail in the future. Unless buying a large tank (100 gal +) I'd probably go new. My LFS is selling brand new 75 gallon tanks right now for $75. Now if you got a complete system, including decent filters and lighting, then I'd consider buying used for a great price. Just my $.02
If ordering a tank online, the shipping usually costs more than what you are saving by not buying locally. Ask your lfs if they'll price match, or at least give you a better deal. Good luck finding a tank!