The less tech-savvy population on the internet is not fully aware of what the term ‘domain flipping’ means. If you are one of them, then this article is definitely a must-read for you. In this article, we will delve into the details of what ‘domain flipping’ refers to, how it is done, and whether it is a profitable part-time business opportunity or not.
In the past, expired domains were used as part of black hat SEO in order to generate backlink sources quickly and easily, and this practice is still used somewhat today. Getting links in this way is not a recommendable practice. All relevant search engines work nowadays with highly complex ranking algorithms, which can no longer be outsmarted by these link networks.
Simply put, domain flipping is the process of acquiring a domain name with a motive of selling it at a higher cost to someone else. This is very similar to flipping houses or cars, the only difference being that you can’t do much to increase the value of the asset by ‘fixing it up’ and then turning it over for a quick buck. Acquiring a valuable domain name before it is tagged with a premium price is the key to a successful domain flip. Raymond Hackney, a domain investor and consultant, successfully flipped a .website domain (his first on a new domain extension) for a decent profit. Read all about it here.
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. 
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.

If you buy keyword specific domains, you're really buying the type in traffic.  I use the URL builder and redirect through that URL so you can see how much traffic your getting from the keyword domain.  There seems to be no rythme or reason to what keyword domains deliver traffic and what don't.  By tracking traffic with the Google URL builder you get a feel for what names are giving you traffic and which are not. ie. the plural, the singular, two words, three words, the possessive, etc.
The second option requires a bit more time and effort than a 301 redirect. You could do a mini overhaul of the site and turn it into a microsite for your main domain. This option is good for exact-match domains for your targeted keyword, and there are other reasons for going the microsite route that Rand's highlighted in his post about root domains, subdomains, subfolders and microsites. This strategy also works better if the old domain has decent rankings for the keywords you're targeting.
Domain investors build a portfolio of valuable domain names, and attempt to sell them at a profit. Both Sedo and Afternic offer domain marketplaces (also known as Domain Listing Services, or DLS) for investors to advertise their domains to potential buyers. Once the seller and buyer agree, the aftermarket platforms then facilitate the purchase and transfer of the domain(s). As an Enom customer, you’re entitled to list your domains for sale on Sedo, Afternic, or both at the same time. Get started
There are many places and ways you can purchase domain names.  You can purchase unregistered ones through sites like GoDaddy.  There are forums where people will gladly try to sell you their domain name (usually though the domains being sold in forums are either crap or the asking price is so high it does not allow you any room to flip the domain).  And then there is my favorite way of obtaining domain names, through an online auction at Namejet.  Let’s first talk about Namejet and then we’ll talk about how I personally ring the register by flipping domains.

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.