GoDaddy’s domain auction space serves as a “marketplace” for buyers and sellers to get together and make deals. When a domain expires, it typically goes right back to the registry. If it’s a valuable domain name, however, registrars will try to sell it through auction, and buyers are always eager to capitalize on potential deals. Participating in a domain auction can be a nuanced business, so take a look at our Auctions FAQ page for more details on the process.
Many non-savvy internet users may not even be aware of the existence of other extensions. That said, because there are many new extensions popping up every now and then, you can often find amazing one word and 3-letter domain names with these new extensions that are otherwise almost impossible to find unregistered in the .com world. Additionally, these new extensions are becoming more and more familiar to users every day, hence it's almost certain that their value will continue to increase as time passes and wider adoption manifests. 
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.