Domain names expire when someone decides to stop renewing it. They may not be available to register immediately. We update the search index every night, so some names may already be renewed or re-registered. Some may become available in a few days. Some registrars, like GoDaddy, will let you buy a name that one of their own customers expire immediately. When you find a name you like, just click on it to try backordering the name!
As .COM is the natural domain extension for the United States based websites, it does get some priority in the Google.com’s search engine results (based on Google’s US datacenters) when competing against other countries’ domain extensions like .CO.UK, .CO.ZA, .COM.AU etc. However, .COMs don’t get automatic priority over other similar extensions like .ORG, .NET etc. that are also commonly used in the United States. All these domain extensions compete with .COMs on a level playing field.
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.