Mark prices on the animals clearly.
Do not badmouth other people, other animals or the venue. Period.
Don't brag that you have "the cheapest animals".
Be proud of the animals you have and don't hesitate to show how much you love the hobby. Take time to answer questions for a person buying a $10 normal just like you would someone looking at your $5000 snake. You never know when that person buying a $10 pet might end up a reliable customer or a link to more.
Hydrate. Take breaks and make CERTAIN that your assisting people also take breaks. If you have friends volunteering time to help you out, appreciate them and do something nice for them as thanks.
Keep the animals clean. Everyone knows that snakes poo but few people want to buy a poo-covered snake.