General:
* Trader
* Retailer
* Vendor
* Dealer
* Shopkeeper
* Businessperson
* Salesperson
* Business owner
More specific:
* Wholesaler (sells in bulk)
* Importer (brings goods from another country)
* Exporter (sells goods to another country)
* Broker (acts as an intermediary)
* Distributor (sells to retailers)
* Supplier (provides goods or services)
* Haberdasher (sells clothing and fabrics)
* Grocer (sells food)
* Jeweler (sells jewelry)
* Bookseller (sells books)
The best synonym will depend on the specific context and the type of goods or services the merchant deals in.