I've never ever liked buying second hand full bouncers - hard-tails are always east to assess, but fs are always full of nasty surprises even if they have new bearings/bushings and a serviced shock.
What would you look for on a 5 year old Tallboy - do they have a reputation for breaking anywhere in particular?
What do the good people of STW think of buying a tallboy which is 5 years old (i'm sure by one careful lady owner who never rode it anywhere but to the shops....which seems to be the case with 90% of used bikes on ebay etc!)
Get the current owner (if they are the orignal) to order a set of bearings, whether it needs it or not.
It takes 5 min on the SC website and they come free in the post in a day or two.
Is it a carbon one? If it is check that the bottom bracket shell and all the pivot bolt inserts are not moving at all. You can tell this by checking the paint and making sure it doesn't have a cracked ring around it.
The 3 or the 2?
The two I think has a weird shock size, so worth making sure that is ok.
Older style high shock SC in general, check the carbon in the area where the suspension axles/pivots joins the frame.
Also a lack of downtube protection so if the seller is somewhere rocky, check for downtube damage.
I've an XL LTc about to come up for sale if anyone's looking...
Mine's a bit worn around the edges but shock/bearings etc all sweet so there is no need for them to go rattly if they're serviced regularly.