Farmers markets (at least locally for me in California USA) are a bit of a crap shoot. They've become popular enough that one has to be discerning because some are infiltrated by people simply reselling goods from warehouse stores, though the boutique varieties of produce are pretty much guaranteed to be authentic and if one is lucky some regular produce is much cheaper than even discount grocers.

Decent vendors usually list their website or farm name on their tent/stall. They could make things up of course, but people who frequent the market would get wise to the shenanigans. If it’s some guy with a bunch of wax boxes worth of produce then yeah, they may just be reselling -just like the rando selling strawberries on a random corner without a license.

Both in Germany and Croatia, that's enforced by the market managers, aka the local government.

Obviously there's exceptions for stuff that's out of season or doesn't even grow in the country - but they do a darn well job at ensuring everyone plays by the rules.