Domain names have been on sale ever since the Internet revolution began during the 90s. Just because some entrepreneurs had the foresight to buy profitable domains before the rest of the world caught up with the trend, doesn’t make them domain hoarders. Domain names are just like the real estate sold in the off-line world. Anyone is free to buy/sell as many properties as one likes if his/her means permit.
There’s an important distinction to make here between domain flipping and website flipping. The latter mainly refers to buying and selling full websites. By full websites, I mean websites that actually have content and more often than not have traffic and revenue. When buying and selling websites, the domain name matters less as the main value there is the content, traffic, revenue, history/reputation, sustainability and growth opportunities. The same can’t be said for domain names where you’re just selling “the name” and hence it’s all that really matters.