Warning: this is long lol 
this is more general tips than what you can sell for the most part
Definitely know your audience! Who’s more likely to visit a market/what do they like and location. For example if it’s a usually hot place then making something like earmuffs might not sell well if it’s a hot climate (this is just a simple example) Don’t worry if you think a certain age group won’t like a certain thing, I guarantee you there’s an audience for everything regardless of age, in general cute sells!
Also keep in mind the price, you don’t want to undersell your work! However keep in mind if you’re only accepting cash that not everyone carries around a lot of money, I personally don’t carry bills higher than 20/a lot of money so having smaller items that’s don’t cost as much is a good idea. Also consider making the same item in many colors as that can get more sells when there’s options. Also it’s not as bad idea to have more big/expensive items in display as well to catch others attention to visit your booth. If it sells, good! If not it still gets the attention of others
Have a great customer service, don’t be on your phone/distracted/quiet make sure to engage with hello’s or ask how they’re doing, how they like the market, etc
I would consider having hand sanitizer as well, just to make sure to stay sanitary (plus I always put on hand sanitizer especially if I want to feel an item like yarn so I would definitely go to a booth just to use it lol )
Stay hydrated! Bring lots of water. I don’t know if you’ll be inside or outside or the climate where the market is but stay warm/cool
Consider looking into/buying items for better organization/displays looking online would help too
Honestly there’s a lot of great advice here, use the magnifying glass in the community and search market and click the topics to see some comments because a lot of what I said I got from those and I can’t remember all the great advice
Good luck with your market! I hope you do well 