If you were buying a 125...

From my point of view, they all look like decent candidates.

Quick thoughts:

The KX sounds like it has probably had the best maintenance program of the 3. If the owner kept logs of his maintenance, even better. The KX was not the best of the 125s in 2005, but a well maintained bike is worth a half-dozen magazine shootout wins.

The YZ was probably the best of the 125s in 2005, but the fact that this bike still has the original tires on it makes me wonder if this bike has ever had even the most rudimentary maintenance. It could be a nice find, though.

Any ad that says "bike is really fast" (the RM) always worries me. Can't put my finger on it, but "really fast" bikes tend to be bombed out roaches from a mechanical standpoint.

Talk to the owners and get a feel for how serious they were about maintaining the bikes, but plan on spending a few bucks regardless. Go see the bikes. If it looks rough, it is rough.

Look on craigslist again.

I just listed my daughter's 2001 KX125 today.

Nice bike, plus some goodies I'm willing to throw in.
Here's the gist of t - 6 speed, two-stroke. This is my daughter's bike, but due to college and boys (argh) she doesn't really ride anymore. This is a great bike. Always starts. Runs like crazy. Just a blast to ride. Not really for beginners, but forgiving enough for novice riders. Updates include a 144 cc big bore kit. Protaper top clamp and oversized bars. Procircuit Platinum expansion chamber with R304 shorty silencer. Comes with the original OEM exhaust and top clamp, service manual and clean title. Will include a helmet and complete set of riding gear and boots (size permitting)

If my memory serves me.. the 05 RM125's were the tits, weren't they? I'd call all of them, see which had the best maintenance program and whatnot. They all look pretty clean.