I bought a new one recently.
My dad pointed out the the main thing to look for is (bare with me, don't know what it's called) the end part of the belt. The circular part, where the belt spins on, it should be really big.
(hard to explain but you know what I mean)
My old treadmill had a smaller bottom frame, new one is much bigger, you diffently notice a difference in terms of smoothness and stability when running. When the rotation thing for the belt (sorry again) is small it breaks, the last one broke.
(apologies for my busted up English today)
Oh, the thing I was trying to explain is called a "roller", it should be big.
I highly recommend this one for 900 bucks in stores:
New Balance 1500
New Balance New Balance® 1500 Treadmill
And like I said, the main thing you want to look for is at least an average size roller, you definetly don't want a small one. You'll notice a big difference when you run on it.
cheers