How to prevent ppl from buying...

In a regular store permit, you require more mesos in your inventory than the shop tax is for a max mesos transaction (64,424,509 mesos). In a hired merchant 1 meso is sufficient. Get someone to buy the 1 meso item *before* you put up your max meso item, and don't "tidy up" the 1 meso purchase.

Confirming this. Store permits collect tax before transaction white hired merchants calculate taxed fees after, so you only need 1 meso in those situations.