Sorry, very busy day here.
We have been doing market gardening here in GA for about six years now. The biggest thing is to know your customers, but since this will be your first year, it may serve you well to talk to some of the vendors who have been there for awhile and ask them what kinds of things the customers seem to like. Growing niche vegetables sounds like a good idea but it just doesn't always work. But, the things you mentioned (long beans, scallop squash, white cucumbers) are not so out of the ordinary that customers wouldn't buy them. I would just include "regular" veggies as well, if for nothing else to attract them to your booth and then they can see some of your more "uncommon" offerings.
The second most important thing is exactly what Greene mentioned, but not necessarily the apron, but the SMILE IS A MUST. And greet them, even if they are passing your booth up. Talk to people. Talk to the vendors. Have fun.
I don't necessarily agree with not having a ton of produce on the table. It can work both ways but most people are drawn to full tables and organizing things is a big plus. I have had many customers tell me that our booth is always the prettiest. Maybe it's because we sell cut flowers too? So, if your offerings are slim, use a smaller table but display things nicely.
The recipe cards are a must if your customers are not typically buying your type of produce. That will make them feel more comfortable purchasing them, knowing how to prepare them. Samples are ALWAYS a huge hit and will help sell a product that would not normally sell. Take white cucumbers for example. Unless people have had them before, they will opt for the same old regular cucumber, but if you slice a couple and give samples while telling them how they don't have to peel them and they will never be bitter, nine times out of ten you will make a sale. And that sale will bring them back, over and over again. Especially if no one else has them.
I find it important to have pricing cards. A lot of people are not comfortable asking prices and will pass you buy. If they can see your prices, they may pass by to check out the other vendors and then come back to you. Also, I highly recommend using a credit card processing service. We use Square. It won't cost you anything to sign up and get the card reader and the transaction fee is minimal. Most people are not carrying cash anymore, even to markets where cash is traditional. If you can take a credit card they will buy from you and often they will buy more than if they had cash. I resisted doing it for a few years but I am so happy we use it now.
If you want to tree mail me with specific questions, I'd be glad to answer any questions and give you suggestions. Remember to have fun!
Having your name on the recipe cards is a good thing. 
