I have been a merchandiser for over 10 years so let me take a stab at this
There are essentially three different types of merchandising jobs
1) Reset work - mostly heavy work involving moving shelves and resetting product to the new layout, including retagging.
*This can be physically taxing at times.
2) Coverage work - this would include regularly scheduled visits to either one store or a series of stores and can include things like checking to see if an item is in stock, stocking shelves (assigned brands only), verfying if displays are up and/or setting displays, education on new products
*in this role you are the eyes and ears of the brands that contract with your employer, not as physical in most cases
3) Project work - this is mostly one time visit work, which can include assignments such as putting up graphics, placing coupons on product, setting up a display
I do a little of all three. In fact today I had a major reset and before I left the store I finished up some project work from another company so I would not have to go back later.
Keep in mind that many companies put in their job description that you have to lift up to 50lbs.