An important addition to your list of items that makes inventory more valuable:
2B) Purchasing intent for a commodity item where the inventory seller can extract a high percentage of the profit.
Apple isn't going to bid up the price of an ad that erodes their profit too much because, at the end of the day, they're the only source of iPhones.
Your local florist will compete against the guy across the street to buy that add even if nearly all the profit goes away because he can't easily differentiate his product.
2B) Purchasing intent for a commodity item where the inventory seller can extract a high percentage of the profit.
Apple isn't going to bid up the price of an ad that erodes their profit too much because, at the end of the day, they're the only source of iPhones.
Your local florist will compete against the guy across the street to buy that add even if nearly all the profit goes away because he can't easily differentiate his product.