Not every store is that easy to deal with as I have seen and heard this discussion inside. Many get straight up angry about it so please do not be Carte Blanche about this. If you know them and it is cool then carry on. If not check 1st. You might be surprised.
Yep. Its actually kinda rude to do business on somebody else's property when its their primary business. I'd be pissed too if you showed up on my property selling one of my potential customers something I sell (or in this case facilitate a transfer for a fee) and cut me out of the deal. At least thats how they would view it.
But as CM said, if you know them and they are cool, thats fine. Just dont assume everyone would be cool with it.