If the sale is to someone you don't know, ask for ID (DL - carry permit would be better if they have one) and make sure they are a resident of the same state you live in. Make sure they are old enough to make the purchase. I'd also make sure I had someone there to watch my 6, just in case a stranger wasn't interested in a lawful trade, if you get my drift.
Other than that, it depends on your state laws concerning private transaction of firearms.
I assume you are in Utah.