In a world where friends, charm person, suggestion, and dominate person are a thing, shopkeepers need to protect themselves from people (aka, adventurers) who would use such magic to cheat them out of hard-earned money.
Some shopkeepers take to hiring skilled spellcasters to place this ward on their establishments.
The spell doesn't stop all mind-affecting magic...just the use of such magic to affect thoughts about money and merchandise.
It's not one-sided though, as it also protects customers from the same sort of magical influences.