When I first started online, I'd register domains that I thought I could develop and sell at a later time. After learning my way around the cloud, I realized many of the domains I'd registered are pretty much useless, or so I thought, and I let them expire. Come to find out several of these domains have been picked up by parking sites and are being offered for sale for thousands of dollars! Now I'm not saying they're actually selling for the asking price, but I learned my lesson. I never allow a domain name to expire. I add content to it, backlinks, some AdSense and try to sell it for something.
2) prerelease auctions.  These are domains where the auction venue has contracted with the registrar, like Fabulous or Moniker, to auction off non-renewed domains.  Just before a domain enters the pending delete process, it goes to a private auction to any person who has backordered the name.  Some people feel that back links on these type of auctions still provide a SEO benifit. My testing indicates that there is zero SEO benifit when these domains are redirected into another site.  So if you get a domain with "pink squirels" in a lot of achor text to a domain and then point it at your domain, you don't rank for "pink squirels"

The market-driven principles of the domain trade mean that a domain is only worth as much as the buyer is willing to pay. It is for this reason that criteria such as market potential and usability play such central roles in determining prices. Values can change immediatly and without any warning. The price of a domain that was once of little interest to anyone in years past can skyrocket once, for example, a newly founded company takes interest in that same name.
The domain name industry is quite similar to the real estate industry in a lot of aspects. There are end users, brokers, consultants and domain flippers or “domainers”. Domain flipping works similarly to buying a house, renovating it (or even sometimes just sitting on it) and then selling it again at a higher price point. The gist of it is: you’re purchasing a domain name and betting it’s worth (or will be worth) more than you paid for it. If you’re right, you get a nice paycheck and move on. Those who make a living out of this just rinse, repeat, and scale.