Anytime you are buying an engine out of the vehicle that can't be heard running it's a risk. There is zero way to validate mileage, compression, maintenance or hear it run. About all you can do is look it over as best you can, make sure there are no glaring issues, see what the oil looks like and get a general feeling based on questions you ask.
This doesn't matter what type of engine it is, the same ideas apply. You just have to use your best judgment.
Pricing really varies on these engines. 6.0's are going to have a higher price normally because it's the more popular choice over the 4.8 and 5.3. One positive is that you can see the size of the engine is by looking at the casting on the end of the block as it will say 6.0L or 4.8/5.3L . Don't get tricked buying what you think is a 6.0 and it's actually a smaller version. Mileage is a good gauge, but the higher the mileage the more maintenance plays into what it could be. LS engines have great longevity when taken care of. I got mine with 200k on it and have put close to another 25k on it. I knew the history on it and heard it run before we pulled it. I knew I wasn't risking anything taking it home. Popping off a valve cover will give you a gauge of how it was taken care of. If it's clean and sludge free it's probably legit, if you see sludge and build up it's questionable.
Part of what you are buying with the engine is all the accessories. Intake, fuel rail, harness, serpentine drive with alternator, p/s pump, coils and all the brackets one will need to put it in another truck. So the more complete it is, the more it's going to be worth. I used an engine we took out of a customer's truck and since we had to use all that stuff on the new engine, I didn't get anything other than a bare long block. So the engine was free but I still ended up spending a bunch of money finding all the stuff needed to complete the engine.