In reply to dankspeed:
I doubt it is worth $70. Depends on brand and condition, of course. But, X-mart bikes have been sold with 18, 24, or even 30 gears for many years. A 6-speed is either really old, or the chainrings bit it. If the chainrings went, it begs the question how the bike was (or wasn't) taken care of.
Shimano makes an extremely wide range of components, from very cheap to jaw-droppingly expensive. If you note the model, a quick search of the interwebs should help you figure out where in the quality scale the components were. Usually that hints at the quality of the original bike. Of course, Shimano frequently changes their naming schemes, so a little detective work may be necessary.
I would guess that you can find something better for cheaper at a garage sale.
In any case, if you buy a well used bike, giving it a quick tune up will probably make it feel like new.
First, I would adjust the brakes. Make sure they work smoothly. That might mean replacing the cables, which is an easy job. Make sure they have good pads. When released, they shouldn't drag.
Second, I would make sure to give the chain a good lube. The bike should be very quiet when you are pedaling -- no squeaking, grinding, or rattling noises. If lube doesn't quiet the chain, replace it (or have a local bike shop replace it). Chains are a consumable (and, in most cases, pretty inexpensive) item. They can last some people 10K miles, I tend to get about 4K before I need to replace it.
I use Finish Line Red (http://www.finishlineusa.com/products/lubechoices.htm) when lubing the chain, but there are lots of good products out there.
Lastly, keep your tires properly inflated. Tires with low air pressure are prone to flats and require more pedaling effort.