I personally say hello and ask them how they are and then I leave them to it.
I sell on Etsy, my own website and had a physical shop until 3 months ago. I've done them in the past and now starting to do some markets/fairs again so I can offer you my advice/experience.
In my experience, I've found that what people buy in person and what people buy online are very different. For me, in my shop and in person events, people have wanted grab and go gifts, whereas online, my best sellers are customised pump bottles, personalised oil bottles and personalised Christmas products. So I do tailor what I'm selling and where. I also check out what other stalls are selling so I can try make sure I have different things.
Look at the areas you're selling in because I find location and area do make a difference too. Smaller events at pubs/ churches/ sports clubs, schools I've never personally found to be very good for turnover. If you're finding a particular type of event isn't working, look at different ones.
Presentation of products is key too. Laying things out on a stall isn't very visually impacting so depending on how you set your stall out, it may be worth investing in some new display methods. Speculate to accumulate.
Finally, the current economic climate is particularly challenging so people aren't spending. I'm down 60-70% on online sales vs. last year. So I do think people are just holding onto their money and spending more wisely.
It's hard not to get disheartened but genuinely, you're not alone. It's a tougher world than usual for small businesses.
Keep going