Save your money until you have enough to buy an entry level bike at a bike shop.

WalMart bikes are not made to ride for extended times. The frames aren't bad but the wheels and other components will break after a relatively short period. Doing repairs to a low end bike like that is like reupholstering your Yugo.

Craigslist is fine if you know what you are looking for. from the two threads you have started, it appears as though you don't. You might get lucky on CL or you might end up wasting money.