1. The point about them not actually making that much buying for $2 and selling for $20 is 100% valid. Amazon fees are $3, plus another $10 or so for shipping, plus labor, box, label, gas, etc.
2. On top of that, the business is risky, as evidenced by the fact that he's stuck with the products now. If you have a 50% chance of getting stuck with inventory, you need to make double the mark-up (actually more to account for purchase price) to cover that risk.
3. Obviously he was somewhat naive about talking to the NYT. The reporter clearly has a lot more experience spinning and extracting unfavorable quotes than he has with resisting hostile questioning. Disclaimer - I talked to the same reporter (https://news.ycombinator.com/item?id=22575940 I wrote about my own experience here), and I've also talked to and met both Matt and Chris at various conferences over the years.
4. If you look at the pricing charts, everything was basically normal until 1-2 weeks ago. There was a small increase which is reasonable with the increase in demand. At that point, there was nothing wrong with buying from stores or liquidators and reselling. The article says Matt bought this stuff up in February, which is before any states of emergency were announced, before demand blew up, etc. I think there's a distinction to be made between people going to stores and buying now, and people who did that a month ago.