You gotta shop around. Check out
eBay. I bought some 1/0 earlier in the year for like $2 a foot with $5 shipping.
Flat rate priority boxes via USPS are definitely the way to go. Get a large flat one, coil it up, stuff it in there, and use a ton of tape so it stays stuffed.
Basically you're going to have to figure out how much you can sell it for per foot, figure out how much you'd be paying for it, then figure out what kind of a profit that leaves you remembering you'll have to factor in shipping (unless you just tack that on top of the price for the buyer, making the math easier for you). Determine whether or not the profits are worth your time and effort. If they're lackluster, consider if you'd want to do it just to be a nice guy and to build some feedback.