getting a mountain bike

Question
I am looking at getting a mountain bike, I have been looking on ebay and have noticed that most bikes now are going for around $350, all used.  
not being too familiar with the performance of a bike is it a safer bet to buy a higher end bike, with possible more ware, or a lower end bike with less ware.  also does the year matter as far as quality and performance, how many advances have been made in the last few years.  most of the bikes are around 98'-99'
 
also I am not sure what bikes are comporable to each other, I am not too familiar with how bikes relate between brands. of these bikes, in your opinion which ones might be comporable across brands.


trek 7000
trek 6500
cannondale F700
cannondale m700
cannondale m500
 
thanks,

I hope to get started mountain biking soon.

nathan

Answer
It is my opinion that if the bikes were good, they wouldn't be sold on e-bay unless through a dealer and then the price would be about what you would pay locally without the risk.  Above all, never ever buy anything on e-bay unless you are completely aware of the product and the condition.  If you are asking the questions you are asking, you don't belong on e-bay.  I approach the question of what to buy from a different direction.  I have a bike/ski shop that I respect and trust.  I talk to them, explain the requirements and they recommend a bike.  I told them I wanted a street bike that was capable of off-road stuff but not teeth jarring mountain biking.  When I ride off-trail, it is usually along decent foot-paths.  I ended up with a Diamondback that was very reasonably priced, perfect for my needs and maintained by the shop.  They changed the tires to something suitable for me and did so at no cost.  They threw in a gel seat too.  I am very happy.  I hope this helps.  Good luck and happy biking.