In a nutshell merchandising is going to a specific location(s) on behalf of the manufacturer/distributor to restock merchandise, reset displays, set up new displays and such. It comes in as many flavors as does mystery shopping, but typically a merchandiser goes to a location on a regular schedule, signs into the store's log as a 'vendor' and goes about their business. Things that are frequently merchandised by independent contractors (or in some cases part time employees) are things like changing out the magazines in grocery stores, refilling sunglasses displays, updating the greeting cards, restocking a particular brand's cosmetics etc. There is a whole other line of merchandising that involves picking up recalled merchandise, so several years ago the merchandisers were busy reclaiming all the tainted peanut butter from store shelves and returns to customer service and pet food that had ground melamine in the flour that came out of China.
Many shoppers move over into merchandising because many of the merchandising companies pay by the hour (including travel time), pay mileage and offer systematic periodic routes. They also find it more collegial to deal with stores as a fellow employee in the retailing world rather than needing to make up lies and tales about needs they don't have (which is stock in trade for shoppers).