From my understanding, a merchandiser is mainly involved in supply chain management. Ensuring a client's order reaches its destination, in the right quantity, in good condition and all within budget/time constraints. They are generally the bridge between the manufacturer (mainly PRC firms) and usually the retailer client who is located overseas. Whereas a buyer helps source goods and is also responsible for ordering and predicting trends for the next hit marketable products. They may also need to travel abroad to source the products. Both are very important roles with lots to learn but also require much OT.
Why do you ask? Are you deciding between the two roles? You can find more info from job ads or better yet why not ask the interviewer in the job interview what the job duties are as sometimes the two roles are blurred. I'm sure they can give you a clearer picture