The inner aisles are usually planogrammed (the retail nomenclature for the diagram of where stuff should be, height of the shelves, how many facings, etc.) according to the priorities of the category manager i.e. the person in store corporate offices responsible for that particular product group. You'll notice that categories and subcategories inside of a grocery store are organized into shelfing sections of multiples of 4 feet (most commonly). Each of these sections is assigned to a category manager, and managers usually have multiple categories. So Category Manager A might be responsible for 16' of soup in one aisle and 20' of canned vegetables in another aisle. (The layout of the aisles and which categories go where is a separate discipline referred to as "Space Planning").
So this space isn't necessarily "sold" per se, but the category managers usually work with what's called a "category captain", an individual or team that represents a dominant brand in the category, and they influence the planogramming process. They are supposedly unbiased and are merely providing expertise and insight on their respective industries, but that's a pretty naive view as they clearly get to advocate for their products directly. However, the strategy of the grocer will determine how much influence they actually have. Wegmans, for instance, has a strong focus on private label so Wegmans brand products tend to get the most shelf space no matter what.
I have almost nothing more to add, but I'll mention that I recall reading about Wal-mart's practice of making shelf stockers responsible for deciding which products get how much shelf space, and even for ordering the appropriate products -- the theory being that the shelf stocker is the guy in the best position to notice which products are or aren't flying off the shelves. (Whereas, according to what I read, at a standard grocery store, the shelf space itself is sold to a supplier, and they get to dictate what goes on it.) And when I read that, it made me think of the triscuits issue, and imagine that what was happening was that triscuit varieties were being stocked without bothering to look into whether they were popular.
If you have further things to say in this area, I will certainly read them with interest.