Most MNs are arsenal-rebuilt, and so you find many is as-new condition. Tips for buying would be mostly the same as for any used rifle. Check the bore, of course. Removing the bolt to have a peek is easy -- open the bolt, pull the trigger and hold it, pull the bolt straight out. Many have been counterbored for the first inch or two at the muzzle, so don't be concerned if the front end is smooth-bored. Check to be sure the safety works. Tip -- the safety is clumsy to work until you know the technique. Place the butt in the crook of your elbow. Reach up to the cocking piece with the hand of the same arm, grasp it, pull it back while the butt presses in to the crook of your arm, rotate it counterclockwise, pull the trigger -- it should not move. Use the same technique to release the safety. Check the trigger pull by dryfiring, if the dealer will allow it -- he should, it won't hurt a thing. The trigger is likely to feel lousy-- that's normal, and can be very simply improved a bit once you get it home.
Once you get it home, even if it looks brand-new and squeaky clean there will be old caked-on and hardened grease hidden in the nooks and crannies. Look online for detailed instructions on disassembling the bolt -- it is very simple, requires no tools, but can be a bit tricky in reassembly. Doesn't take long to learn the procedure, though. A common "fault" is sticky extraction. Virtually 100% of the time, the old grease is the culprit. The stuff they use hardens over time in to a really tough varnish-like substance, and while the chamber will look clean, it won't be. A solvent combined with a good brush chucked in to a hand drill will get it out. Don't be shy about this -- it may take quite a while, and several sessions, to get the chamber really, completely clean, but once it is the extraction problem will go away.
That's about it!