Like any other form of gambling, the simple rule is not to risk more money than you can stand to loose if it all goes wrong. You could spend a lot of time previewing and still end up with a hidden failure that would cause you grief as soon as you got the truck.
The flip side of this is it might go well - and you'll end up with a nice truck for a good price.
I've got one truck from GL, and another on the way from a private sale. The GL truck is nice - but had a couple safety related issues I'm addressing in order to get an inspection sticker. The one from private sale has been gone through completly, and is as ready to go as a 38 year old truck can be. The difference is price - you'll spend twice as much or more for something that has had a significant amount of risk removed.