Contentious issue: I do not agree with selling and promoting dictators and their tee-shirts.
I would simply walk in, tell her/him why I will not buy anything from them and walk away.
Vendors simply want publicity and attention and more business and not knowing anything (as most Thai know very little about outside world and its history).
It is her ignorant right to sell, but it is repulsive to see hitler, stalin, putin, franco, pol pot, mao, etc…on a tee shirt and someone trying to promote by selling , very insensitive.