So, with the classic client, when you left click one time on an item in a vendor's inventory, it gives you the option to buy. Unless I'm overlooking it somewhere, I can't find an option like that with the EC. So I resort to using "vendor buy" phrase. But when the vendor is on a rooftop, as are most vendors, I get "Target can not be seen." If I can't access the players roof and get close to the vendor, I end up having to switch clients.