When to send it
On market day, at the stall. Stand the printed table card next to your card reader or display, where people wait while you wrap their purchase. Browsers who don't buy can scan it too, and their answers are often the most revealing. Mention the online code as you hand over a bag: scan this and you'll get 10% off on our website.
Why these questions
The first question asks how people came across your stall: walking past, following you online, a friend, or the market's own listing. Over several markets that tells you whether a pitch earns its fee through passing trade, or whether your own followers would have found you anywhere.
Next, did they buy something today? Anyone who didn't sees a short checklist of what stopped them: price, not finding what they wanted, the size, colour or flavour they wanted not being there, not being able to pay the way they wanted, or just browsing. Stallholders rarely hear these reasons, because people who don't buy simply walk on to the next stall.
Everyone then rates their visit on a five-star scale, and the last question asks what they'd like you to bring next time. Requests for a product you left at home, or a smaller size for gifting, are easy wins for the next market.
The coupon
A shared code makes sense here. The point of a stall is often to meet people who go on to buy online, and 10% off your website gives every visitor a reason to take that step. One code for everyone is fine, because the QR code is open to anyone walking past and the discount only applies on your own site. Visitors receive it on finishing, whether they bought something or not.
To see which online orders came from which market, change the code in your rewards before each market day. Your shop's discount report then shows the orders each market brought in.
What to do with the answers
After each market, note the stall rating and the top reason people didn't buy next to your takings for the day. Plenty of browsers with "price" at the top suggests a smaller, cheaper line for markets. If "I follow you online" grows over time, your posts announcing market days are working.
To learn more about the people who then buy online, the who are our customers template is a good next step, and where did you hear about us covers attribution on your own checkout.