He works in retailing industry(Walmart,Target,Amazon,etc.), he decides what products to sell(purchase) to the customers, and what products to be put on the promotion list, and what kind of promotion activity to push the sales.
I agree with Cagey's distinction between "merchandising" and "merchandise". I also agree that 'Merchandising Manager' fits the activities Johnny has described in #5.
But I differ regarding the EducationPortal.com statement that merchandising managers are often known as purchasing managers or buyers. This may be an AmE/BrE difference. In my experience (which seems to be supported by a number of UK ads I've seen), a merchandising manager's responsibilities largely cover the selling aspect of retail activities (including pricing, promotion, presentation, etc) as well as decisions on what the product range should be; but they don't usually include the actual role of purchasing, which is a different function with a different skill-set.
A merchandising manager's role, as I know it, forms a bridge between the buyers on the one hand and the front-line sales people on the other. But as I said, there may be an AmE/BrE difference.