In order to maintain ownership of a domain, you must renew it yearly. If you don’t, it “expires”. An expired domain goes through several stages before being released again for general registration. The process differs slightly from one registrar to another but it’s more or less the same. GoDaddy explains what happens to domains that expire with them here.
Hundreds of thousands of domains expire everyday. While most of them are trash, quite a few are aged domains having excellent backlinks. These can be easily filtered using metrics from Majestic (Trust Flow) & Moz (Domain Authority). These are great for your money sites or for building private blog networks. Some of these domains have natural organic traffic, that can be determined by their SEMrush rank or the SimilarWeb data. These are great for domain parking or for promoting affiliate offers.
There are many different ways to buy and sell domain names. Many standard domain registrars, such as GoDaddy.com, will facilitate sales via auction. Meanwhile, specialized websites, such as Sedo, are designed exclusively to buy and sell domain names. Finally, parking domains with a for sale page is a great way to draw targeted interest from potential buyers.
In theory, you could technically acquire a domain for $10,000 and be able to sell it for $100,000. You could also acquire a domain for $10 and be able to sell it for $100,000, $10,000… or $100, you get the picture. Because of this, it’s hard to recommend a perfect starting budget because your strategy, experience, and the pace at which you learn will hugely affect your buying and selling decisions.