Since this is a diamondback forum I thought I would ask here. I want to buy a mountain bike, I have never ridden on trails so this would be my first mountain bike. I saw this one for 200 is this good?
Looks like it's a 2003 Topanga Comp Disc
Here's a link to specs (appear to be the same):
BikePedia - 2003 diamondback Topanga Comp Disc Complete Bicycle
Price is hard to say - $200 is a fairly big percentage of the original $549 price considering it's eight or nine years old. That being said, it may not be a bad deal for an LX-level bike with mechanical discs if it's in really good condition.
1) Make sure the frame doesn't have cracks or major dents. Cracks are usually spotted around the welded joints.
2) Make sure the fork is working well (smooth operation through full travel, adjustments work and feel like that make a difference, etc). Note: The fork is the most outdated item on that bike, but is likely fine for entry-level cross country riding. If you have to replace the fork, though, you'll spend more than the price of the bike if new, or you'll need to do some searching for a good, used one.
3) Make sure the hubs, bottom bracket and headset are smooth and free of play and that the wheels are reasonably true (no large hops/jumps/rim dents/broken spokes).
4) Very important - make sure the bike fits you! Search online for guidelines and make sure it feels good to you.
If you don't work on bikes and don't want to learn, you should seriously consider having a shop check it out. You'll pay a small fee for this, but it's worth it.
With all that done, I'd offer less - maybe $150, but, again, condition matters. If the owner has recently had it serviced and has a shop contact, that paperwork is worth a lot to you...
At $150 and with no recent evidence of work done, don't be surprised if you need some or all expendables: cables, brake pads, chain and cassette.