As I have experienced it myself and it has become META - people love to invis gank kidnap/FD/Grief their enemies when they see them open a Stall because they know the person will be distracted and unbuffed.
I suggest the message a person has opened a stall be removed to reduce this obnoxious, abusive PVP tactic.
The intention of the message is to inform people that the stall has opened. This is for advertising purposes, it is also entirely IC — and the potential to attract criminals is also something the DM team views as a positive.
It is not obnoxious, abusive, or meta for someone to rob a merchant after hearing their advertisement.
If you do not like the idea of being robbed, hire bodyguards.