Thanks for the replies.
Yeah, just *filthy*, isn't it? My LBS has a big ultrasonic cleaner and it's so bad that I'd be tempted to pay them to throw it all in (except the frame), rather than spend the countless hours cleaning it.
I rode it last night. The tires were under-inflated, so the bike felt very sluggish. Rule 1 selling a bike: have the tires fully inflated or it will feel like a pig! It totally killed any wow factor of an extremely light bike. It felt more sluggish than my rather heavy K2.
Also... It had a somewhat noodly feel. That was hard to isolate with the under-inflation. But when riding on the road, on the seat, and laterally flexing the bars side to side, it seemed to have a lot of flex. I'm not talking hard torquing like you'd do when pedaling hard - just gentle efforts produced the feeling of flex. I am inexperienced with carbon frames, so I'm not sure if that is typical.
Anyway, I figured the dirt in those photos would inspire a major negative reaction. It is painful to look at that. That's stuff that could have been hosed off. The thing about dirt like that is it traps moisture day in, day out with the humidity changes in a garage while the bike is stored through the seasons. That trapped moisture leads to corrosion, and corrosion tends to progress. And if there is any road salt in there, oh boy.
The bike was described as "great shape". The rear shifter was gunked up to the point the lever wouldn't return - you had to move it back with your fingers. The bike was only 10 minutes away, so it was no big deal and well worth looking at. It was a good learning experience. Just like used cars - if you don't go and look, you'll never learn what to look for. A friend and I would go look at cars at lunch, and we made a game of who would spot the evidence of major collision damage first - and they almost all do.
I have very different definitions of "great" and "excellent". A "great shape" bike should obviously be ready to be ridden, and well cared for.. and no hub slop (rear hub also had significant play), or BB play, or forks covered with oil (the manual says they leak if the bike is hung, so that wasn't necessarily a huge deal - they felt okay).
Anyway, after the ride I laid out a large piece of cardbord to sit on and started inspecting the frame with a light and a magnifier. I wiped it down to get a good look. I noticed a straight line around the BB. Probably where the BB lug ends. It was fairly smooth.
More concerning were the apparent cracks on the top tube near the seat post. Those seemed to have some depth to them - the paint was flaking and the gap in the paint growing larger. If you look at the detail in the large photo, those look quite bad to my novice eyes. The quarter test was uncertain - there was some dullness to the sound on the crack itself, possibly due to the paint flaking.
I did not flake the paint away, since it was not my bike. But at that point I was done. I did not continue the inspection to other areas. The anxiety over those apparent cracks would take the fun out of the bike, and I don't think I'd be able to enjoy it. Maybe they are a non-issue, but it was not for me. I'm 190-200 lbs, so not huge but not light.
The k2 isn't a bad bike and it was a decent buy for $90. I've rebuilt the bearings, new brake cables, pads, and chain. But when I "launch" off a 1' drop, oh it feels like a very big deal to those forks and wheels. It is not the KDX 250 that I once raced and jumped with abandon.
So my plan is to ride the k2 for now, trying to not sink any money into it, and keep my eyes open for good used bikes. I'm in the process of growing a business, so money is tight.
There is a Trek demo event this weekend, so I'm going to check that out and get some experience with modern bikes and 29er's. I'm expecting it will be a hugely useful learning experience and help me narrow my focus and plan. I have a lot of questions right now over sport, trail, freeride types of tradeoffs vs. weight. And also frame material.