It differs from state to state so check with your coopertive extention in Texas. Also farmers markets have different rules so get a copy of there rules as well.
For example at our state run market you can hire some one to sell for you . The small specailty markets I sell at the grower has to sell.

I totally agree...check with the market manager. He/she should be able to fill you in on what you will need in the way of permits for selling at a market. We sell dried herbs and these are considered "processed" so we have to carry liability insurance as well.

Our market is inspected by the local health department a few times during the season.