Haven't tried selling stuff myself but there seems to be a few tips in this thread. Quoted two posts.
"There is a table for objects, a wall for artwork and a tarp for big objects. These can fit in your inventory and you can sell anything on those from your inventory. For example, all your snow globes you've collected. "
"Click on an empty table or wall and choose "open inventory". Then open your sim's inventory and drag their items into the table/wall's inventory. "
https://forums.thesims.com/en_US/discussion/904550/how-to-sell-stuff-at-the-flea-market
Good luck! I will swing by later, the kid of my current sim will be a crafter so this is interesting to me :)
