I thought of another benefit to clearing a firearm before handing it to another person that I witnessed at a LGS just yesterday. If the receiver is very unfamiliar with firearms and how they operate, you just showed them a good habit and how to do something in one simple motion. This dawned on me when the LGS clerk handed over a Ruger LCP to a customer without clearing it and the first question from the customer was "How do I operate the slide?"
I've always (since I started looking at buying guns) understood why you would want to clear a gun before you start checking it out. However;
-When I bought my first revolver, I didn't know for sure how to open up the cylinder. (yes, I know, pretty obvious, but it was my first gun, ever.)
-When I bought my first semi auto pistol, I didn't know if there was anything that needed to be released before opening the slide to check it.
-When I bought my first shotgun I didn't know where the release was to release the slide to check it.
-When I bought my first bolt gun I didn't know that you had to put the safety off to open the bolt.
Semi auto rifles were about the only gun that I really knew how to operate properly before I purchased one.
So yeah, in a gun store, you better assume that the guy doesn't know how to check one, and you had better check the gun before handing it off. I've had way too many guns pointed in my direction with fingers in or almost in the trigger guard for me to be ok with the store employees not checking a gun before handing it across the counter.
I got annoyed when I went to a Dick's sporting goods and asked to see a certain gun, the girl checked the gun, and then apologized for having to check it, it's just that her manager makes her do it every time.... If you don't understand why you need to do that, maybe you don't need to be working the gun counter. Stores like that have ammo sitting right there, and pissed off employees do stupid things. I'd hate to see a gun "just go off" because someone put ammo in it and someone else assumed it was unloaded. If a gun store hands me a gun without checking it, that's my last time in the store.