So many questions, I'll try to answer what I can.
If you're hanging out with your friends, then you're probably partying? Partying while vending is a no no, since you're could potentially use @at and not be there. Which is where the objections come from.
One of the other reasons is the one Zerrif spoke about. A merchant on a map, will stop a map from resetting, when someone goes away after luring monsters in a section. That is a no no. I found a player who had done that last year I think...
Say you're partying and you have really nice stuff in your cart and your friends need that healing pots to continue partying, I, Aki, wouldn't object to you doing a fast 3 minutes shop to sell them it. But more than that? It is hard to trace the line.
I feel like I answered in a most disorganized way
Still, its a start no? =)