Used Kid's Bike--what do I look for?

My son has outgrown his 20" Trek mountain bike and we found a 24" Trek Mtn 220 today at our local Trek store. I think it is a 2010 bike and is selling for $150. It seems to be fine, but he didn't have time to ride it. What should I look for? I'm thinking: Do the brakes work? Visually inspect the pads as well as have him ride it. Does it shift smoothly though all the the gears? Anything else?

Unfortunately they do not take trade ins when people buy used bikes. I'm wondering if that is a firm policy or more of an idea. Any tips for bargaining?

Thanks!

Check cables for being frayed, etc... look at the frame just to make sure it's sound (but a bike shop should have done that kind of lookover).

I volunteer at a bike coop... trade-ins are a can of worms we don't go near. We don't know where that bike's been or even if this is, shall we say, the original owner...

Thanks Muirren and Geonz!

The lad has a new (to him) Trek 220 and they did take his old bike in for a trade. :)