I am looking at a used Trek 850 MB. I am a very new rider and want to see how much I am going to actually ride before spending any major $$ on a new bike. This used Trek 850 is in pretty good shape (to my newb eyes anyway).

Can anyone tell me, as a first-time used bike buyer, what I should be on the lookout for? The bike is visually in pretty good condition and the shifter action seems to work pretty smoothy. I know it will need a good tune up but if there are specific things I need to check out, I would appreciate any advice.

BTW, seller is asking $100 for the bike. I am not sure (nor is she) what year model the bike is.

Thanks people!