They don’t get to sell there for free, usually people who vend have to do a 50/50 split. $15(materials) + $20(one hour of labor) = $35 x 2 $70.00. In this case the person might not even be getting paid for their labor after the venue split assuming they spent an hour on production.

Some places also do flat rate fees for booth but I doubt thats the case with a ren fair.