Buying used can be crap shoot! If you don't know what you're doing, you could get quite easily duped.

It's always best not to gamble. You could do much better with an online distributor like Bikesdirect, Amazon, Nashbar, or Performance.

Currently, Amazon has the Marin Muirwoods 29er on sale for only $450. Since it's size 17", it should be perfect for you!