Hello
I will quote here what Simsvip wrote about trading, and it may help you
Trading – This is the perfect chance for trading off duplicates of extra inventory items. During the Flea Market, Sims can use the “Ask to Trade Collectibles” interaction to select any qualifying object in your inventory. The object you give in return is the same type of random collectible. Sims can trade the following types of items:
Crystal
Fossil
Frogs
Metal
My Sim Trophies
Posters
Snow Globes
Sometimes the vendor will not be in the mood to trade. Your Sim can improve the relationship with the vendor and change their mood to increase the chances of successfully trading with a vendor.
https://simsvip.com/sims-4-city-living-guide/#Festivals
You may not have had an elegible item in your inventory at that time when the trading option was greyed out.
