Here are some options you could use, depending on the context:
General:
* A group of merchants
* A gathering of merchants
* A company of merchants
* A body of merchants
More specific:
* A guild of merchants (if they are formally organized)
* A bazaar of merchants (if they are selling goods in a market)
* A cartel of merchants (if they are working together to control prices)
* A trade delegation of merchants (if they are representing their business interests)
Ultimately, the best choice depends on the specific context and what you want to emphasize.