I've bought and sold a couple of mamba's in the last few years. This is a good deal-- its probably worth about $400 to $450 without the paddle (which isn't a great paddle but ok for beginners -- its probably worth $50.)
If its in good shape and fits your size, snatch it up. Its a good boat. Its not a ridiculous deal, but a good deal, and the bigger factor that good used boats that aren't seriously flawed (by design or condition) are hard to come by.
If I had a 120 pound person in my family that wanted to start boating or needed a river runner, I'd be all over it.