Will a used car salesman lie to me?

I'm working with a used car salesman right now to find myself a car. I'm looking for a Neon, perferably 2000-02. Friday when he go to his car auction, he's going to try and get me a Neon, hold it until I look at it and let me trade my current car in. Here's the thing, even if the car looks nice and financing it will be easy...I don't want a car with an major mechanical problems: Bad Trans, breaks, oil leaks, etc. I can easily ask him but will he really tell me "Hey, this car has a bad trans". Or will he tell me the usually line of "It runs great!" Because I've gone to quite a few used car lots (Just "window-shopping) and I've asked about any problems on a potiental car and have always been told "It runs great!" Never a real reason as to what makes it run great like "New engine, etc etc." Just simply "It runs great" Which leads me to believe they aren't telling me everything.
It's one of those buy here pay here lots so if the trans is bad and he lies about it, then I'm screwed.

Good news is my uncle is a mechanic and he's willing to go with me Friday to look at any underlying mechanical problems. Only problem is my uncle lives further from me than the car lot so my SECOND QUESTION is: Will he let me drive the car to my uncle's garage to check out the car? By law, or something does he have to?
And I know my question header probably sounds nieve, but I'm not good at wording questions and I guess what I really want to know is can they actually leave out important mechanical issues when they sale you the car?