As long as the phone hasn't been tagged lost/stolen in VZW's system, there is nothing to prevent you from buying a phone from someone and doing an ESN change.
The only thing that might mess things up is this: when you buy from Verizon, they put a pending ESN change into the system for the number the phone was purchased from. This makes it tricky to activate the phone for another line until after the phone is activated for the one it was purchased for (as you may have noticed, you also have to accept TOS when you activate the phone for the first time).
In my case, I bought the Incredible using my wife's discount. I had to activate the phone for her line before VZW could swap the ESN's.
EDIT: Is there any chance the folks who just want to snipe at each other can do it somewhere else? This isn't a thread that should have lead to name-calling...