You have a 2010 bike with 4500miles, and your surprised that your getting poor trade in offers! Bikes especially Full Suspension have bad depreciation, and 2010 is old in FS terms.

For the chain, depending what condition you ride in, 3 changes since you have had the bike is probably too few, MTB chains don't last long, although that all depends on what conditions you ride in.

For the wheels, for the Mavic & Shimano ones, can you get spare spokes, if not, avoid; with any wheel, what condition is it in, MTB wheels can have a very hard life, and you may want to look at the rotors, the XTR are centerlock, so you would need adaptors / new rotors.