I am not sure if manufacturers will be open to selling directly to someone who is not buying a large enough quantities. But even if they are, they probably won't sell them to retail arbitragers.

I cannot remember if it was from the same episode or another Planet Money episode, but I remember them talking to manufacturers who were unhappy with their products being sold online for prices much different than they intended to.

The gist of it is that, if you are for example Coca Cola, you want people to think of your products as an everyday item they can drink anytime, not a luxury to be saved for special occasions. And if you are Apple, you won't want your product to end up in the bargain bin. Pricing is part of a brand's marketing strategy and identity. People who price products way out of the range the manufacturer envisioned are detrimental to their brand image and long-term market share.

Small-time individuals don’t get the same sorts of deals as major retailers.

For some products it can be cheaper to buy retail. The retailers use their buying power to get a lower price than anyone else gets. There have been incidents where all the mechanics go buy their oil from Costco, or all the bakers go buy their butter from Walmart. The savings were enough to justify it.

You can generally skip the sales tax if you have a business license, or there's some way of registering purchases as being for resale (seems to vary by state here in the US).

It could be because conditions change too rapidly. Whatever you are buying and selling this week, it might be different next week.

I know someone who sells a lot of stuff on amazon. It's mostly books, but they do a little retail arbitrage type stuff too. Sometimes it can be as simple as a store in their area having bought too much of something, so they put it on sale, whereas a store in someone else's area didn't, so there's the price difference. Or it can be a discontinued product that is still in demand but stores in one city have extra whereas stores in another city are out.

The whole point of arbitrage is that you identify and exploit price imbalances. But imbalances tend to balance themselves out eventually, so it's often a temporary situation.