Studies show if you give people things they'll be more likely to give you something back in return.
Think of when you have ever received an email from a business that gives you a small voucher on your birthday, would you spend $10 to make $50? of course you would, but just because the gradhack isn't profitable exercise doesn't mean you can't still apply these time tested principles used my many businesses and sales people.
Make sure that at your stall you have something to give, a personal gift that you give to the customers that you can give to the customers that come in. It doesn't have to be a physical thing like a pen or a badge or some kind of treat that you give away, but it can be another kind of gift, a fun fact, a delightful experience at your stall, something that is personal and is a gift that you can give to someone to so they can spread the word about your business.
Ultimately if you can get customers coming in because of word of mouth or because you have something to offer you will have more people to pitch you sale to, and therefor you will have more sales.
For how to sell your product make sure you watch the others videos in this series on how to sell stuff in 90 seconds.