I’ve only been to one market, but these are my recommendations:
Jellyfish - Just make a random jellyfish, and people love them, but make sure that you show the tentacles <3
Bees - this pattern works up super fast, and the little kids love them so much XD
Mushie pops - this one is a really nice pattern, and the pattern is by Cable and canvas on Youtube
Dragon - There is a tutorial on youtube for this, and it’s by crochetbygenna, and it works up super quickly <3
Chicks - They are so easy to make, and so derpy 
Penguins - So cute, and they’re pencuins XD 

Turtles - I made this pattern
, but people say that it sells out really fast <3
Whales - This one is so easy to make, and people love them <3
Links (patterns on ribblr)
Bee: FREE mini bee pattern: Crochet pattern | Ribblr
Chicks: Free mini chick pattern: Crochet pattern
Penguins: Free Crochet penguin pattern
Fruit turtles: Free Crochet Fruit Turtle Bundle
Whales: Free Whale Keychain
Bring a friend
Tablecloth!!
Tiered displays
Labels!
MAKE AN INVENTORY SHEET WITH PRICES (helped me soooo much!)
Signs! (Sometimes, people are shy, so having signs for the price of a specific item helps)
That’s kinda it
, but if I get more ideas, I’ll edit this post as needed 