I'd keeps eys on Crag's list. While I've never looked for treks MTB, and or specifically an 850, as I have feelers out for their 80's road bikes, I see em all the time. Just a quick search an 850, small frame is up.
Just a few year ago, their MTBs of any style or year were tough to find under a buck, but this and last they broke the $100 barrier (buck) and run 25-125 now.
I offered to a lady who was shopping at wally-world, about 3-4 year ago, for a bike, to keeps eyes out for a lady Mtb for $75 on C/L vs the crap she'd end up wit. Then, 3 or more year ago, only every three months r so, but now? They're everywhere. There may be a few hunting for earlier models, like you but as a rule, they'll drop into the same place all other older trek Mtb's are.
Just a little patience was what I offered that lady as, they'll pop up there in due time. .