It depends on what kind of crowd will be there, lots of kids or young families, mostly adults, or seniors. With lots of kids expected, it’s fairly easy - cute animals, or recognizable characters! Mostly adults, you can go for more decor or useful type stuff, or gift-able makes. Mostly seniors, they enjoy cute kitschy stuff usually…stuff like granny square birds lol (those are fun to make!). But even then, it’s hard to really predict what will sell well. It’s usually the things I didn’t expect to sell that go first, rather than the stuff I made sure to have and stayed up late the night before to finish 
You can’t go wrong with some nice fall themed stuff, and it’s probably a perfect time to start sneaking in Halloween too (or at least some nice pumpkins!). In the past I’ve tried to theme items to the specific market, but they never seem to sell. Also, be sure to have some small, quick, cheaper items, so that everyone can buy something nice, even if they can’t afford much. 
As for set up, make sure to have height! I like to put my items in order of price range, more expensive up top, the cheaper on the table, and the middle range, well, in the middle lol! Use a solid nice colour or soft patterned table cloth so your items have a nice backdrop that doesn’t take away from them. Make sure to have lots of clear signage, and business cards or a QR code for people to find/follow you!! 
Be friendly (obviously), but don’t force a conversation, as it drives some folks away…give a nice hello, and let their response dictate whether they’re someone chatty or quiet! (I also find sitting is less intimidating to potential buyers than standing)
Overall, it’s honestly getting harder to figure out what will or won’t sell. Even if you don’t sell much, know that it’s likely not you or your prices (unless they’re outrageous lol), it’s just sadly not that every market is a smashing success.
(Lastly…bring something to work on! Folks love to see the creator at work! )
