I mean yes, initially it is risky, but perhaps a contract was signed, or perhaps the oyster owner owns so many that he sells to local restaurants or market vendors that he figures worst that happens is i lose 2% of my regular oyster haul, best that happens i make a bit more money off of that 2%.

If I am not mistaken the actual episode(s?) where Arya does the oyster selling show a relationship between her and whoever she's getting the oysters from.

As far as real world examples: anyone who has ever been hired to drive a cart or wagon or car could just attempt to make off with the vehicle and/or its belongings...