Yamaha,
As you are sitting on the bike, the flywheel cover is on the left side. It should have 4 or so 8mm bolts. Lowest part of the engine, the crankcase. Round cover. Just guessing, but I believe Eric suggests looking there after you remove the cover to check for any residue or puddles which would indicate a crankseal leak (air). The flywheel contains your stator and timing adjustments. It should be clean in there. If there are any signs of puddling, add a left crankseal install to the price of a bike. It only takes 2 minutes to remove the cover, and will tell you a lot.
Also, you should have no play if you attempt to move the stator up/down right/left, if you have play, the bearings will need replacing.
Everything eventually wears out and has to be replaced, especially on a mud machine. Just try to be as informed as you can. I see no problem with buying a bike that is 2 or 3 years old. Like Eric says, the racers most often maintain their bikes the best. They have to and also have the knowhow and/or resources on upkeep. I just ate an eggroll (shrimp I think) and it really hit the spot.
The bike should have no leaks at all. Before I bought mine, I called on a bike and found out where it was at. I showed up a few minutes early and saw the bike sitting in a garage (commercial) with a puddle underneath it. It was only a year or so old. When I returned, the seller had cleverly moved the bike to the outside of the garage. It had a crankseal leak. Though he would never admit it.
I've seen in general, if it looks sounds and feels right, it is usually ok. new means nothing to me and I've bought plenty of new vehicles to know.
Strongest suggestion is to bring along a buddy who has experience to look at it and ride it before you buy it.
Goodluck and welcome to this somewhat expensive and unusually fun sport.
Sincerely,
Chief