In my deep, dark past I once managed a retail store. Occasionally someone would walk in with a product in hand, asking about placement on store shelves. I had no choice but to give them the contact info for the buyers at corporate. We were simply not allowed to sell anything that didn't come to us from the distribution warehouse. I could understand their position. Every product in the store is backed by the reputation of the chain. Not only that, but a customer expects that if their friend found Item A in store 42, then they can go to store 142 and find the same item - or at least have the manager order it if they're out of stock.
If someone had placed their product on the shelves and walked out, I would have treated it like any other lost item and placed it in the back room or underneath the counter until the owner showed up to claim it. I suppose the person with the product would have sent their friends in asking, and I'd have truthfully told them that we didn't stock such a product and there was nothing like that on our shelves. And if they brought the product up to the counter and asked to buy it, I wouldn't sell it. It's not in the system. I wouldn't have an official way to record the sale.
Even if I did have the leeway to stock extra items, I still wouldn't sell it without some sort of agreement in place with the producer of the item. And if they'd pulled the above stunts I'd be so ticked off at their underhanded tactics that I wouldn't want to deal with them.
This is why PA (and self-pubbed) authors have a better shot at local stores. I'm not knocking that at all. In fact, I've got some photography on display at a local coffee shop. Smaller businesses have more flexibility in terms of what they can put on the shelves.