He didn't just start doing this last week. This so-called "retail arbitrage" is a normal thing throughout online marketplaces, and has been for years.
Sometimes, someone (like me, as a consumer) won't feel like going to the store, and will happily pay ten bucks for what's usually a three-dollar item, because it's shipped to my door. FedEx is the real winner, but guys like this are making a living enabling my laziness.
In other situations, a product will be weirdly short in one part of the country, but available elsewhere. For instance, in a hurricane, a bunch of unprepared people suddenly decide they want some product, but the local stores are sold out. There's lots of that product _already sitting on shelves_ elsewhere in the country. Those retailers aren't going to pull it themselves and shove it backwards up the distribution chain to be redistributed to storm country -- that's just not how distribution works.
So, folks like this will go buy it, off the shelf, and offer it online. Where again, someone decides it's a better option than going to the store. And the guy doing the shipping is doing real work, I don't see anything wrong with him putting food on his table with it.
In all these cases, the retail-arbitrage folks aren't depriving anyone of anything. They're not clearing out the last of anything, and the stores they shop usually do restock the next day, because their local distributor has plenty of product.
This time was different, and that's obvious to everyone now.
But I do find it at least half-plausible that this guy might not have been thinking of it as different. It looked, from his view, just like every other opportunity where he's been making money the last several years, and which hurt nobody.