Usual Holiday Griping: 2017-12-20
People do this all year, I remember all summer people getting worked up about fidget spinners. Now around the holidays people spend a lot more money, so people sell a lot more. It's how people make a business on the side, if they weren't able to find a need and fill a demand they wouldn't be in business long. Buy low, sell high, how it's been since commerce was developed thousands of years ago. People just get worked up more around the holidays because of the "Children" logical fallacy, where morality is suddenly exponentially worse because children are involved in some way.
They do research on what to buy, how much to sell, where to buy from, they still hold stock in risk, if it doesn't sell that's an investment they lose out on. Plus they have to deal with random people judging how they make money on the side. Better than a lot of the MLMs IMO, but everyone does what works for them. If the price doesn't agree with you just move on, like you would inside any store, you wouldn't go to the manager yelling that their supplier sells it to them cheaper.