You can turn then into content sites, set up an actual website, or try to sell that site elsewhere. You may even luck out by selling a site that's up and running and full of great content already. In this case, you can ask a higher price. Or, you can set up a shared hosting or direct hosting website. Shared hosting and direct hosting websites can be profitable too.
If you buy a domain from a 3rd party, the backlinks are valuable.  I always wait a few months before redirecting, so just set up a quick minisite until two page rank updates have passed and then go ahead and 301 redirect.  However, it seems that you'll get the link juice from the back links, the anchor text is ignored by Google.   Buying a domain at Godaddy TDName expired auctions, seem to not count as a dropped domain if you immediately set up a mini-site.
A good majority of web developers view the domain flipping industry with disdain as they face a hard time finding the right domain names for their own development projects. Domain flipping is also looked down upon by corporate and business entities who register multiple domains related to their trademarks or business names. Actually, it is these people who’re the real domain name squatters!
If you buy a dropped domain from Snap or Namejet, the backlinks seem to be worthless for SEO.  They are valuable for traffic if it's targeted to your site.  Go ahead and 301 redirect into your site because it's the traffic from the back links that is worth something.  I use the Google URL builder to redirect these names so you can see the domain the traffic is coming from.
This is a little bit on the grey hat/black hat side of the spectrum. But the fact is building a blog network with expired domains flat out works. The only issue is getting those cream of the crop domains without going broke. This video will walk you through tested strategies and techniques you can use to find powerful, yet affordable, expired domains.
Parking the Domain Name: When you park a domain name, every time someone enters IHateCold(.com) into their web browsers, a page with a few ads and sales info is shown. So you’re letting the visitor know that the domain is for sale and you make a few cents on the ads displayed for views. There are several domain parking services for when their buying and selling domain names, but I’ll go into that in another post.
This is similar to house flipping, where a home is purchased and fixed up in order to sell for a quick profit. Unlike house flipping, however, there is essentially nothing to be done with the domain name in order to increase its value. Therefore, the key to a successful domain flip is being in the right place at the right time in order to acquire a valuable domain name before it is given a premium price.
To sell a domain name, one should learn to price it. Many sellers fail to sell names in the marketplace, simply because they overpriced the domain, and thereby lost the chance to sell it. A lack of knowledge leads to erratic pricing. Experienced buyers will not try to bargain, unless you have a very rare name. A seller should therefore, understand how good the domain name is. What price will he get from a buyer? What suffix does it carry? For instance, .com names are likely to sell fast and will bring greater profits than, say .info names. Similarly, .net, .org and .in domain names are the best to make decent profit.
Whether you can use an existing backlink profile for a new web project under the available domain depends on whether the link target of incoming links can be reconstructed. In order to analyze which content was provided under a domain in the past, you can find out on the Internet Archive Backlinks have no value if they lead to nowhere and this can even have a negative impact. They are possibly redirected to similar subpages or homepages using 301 redirect. In this case the most part of the link’s power is lost.
In addition to the quantity of incoming links, the quality of them also needs to be accessed. The main issues here are: where does the link come from? What kind of link is it? A high quality backlink profile is usually made up of different link types such as footer and sidebar links, links in comments, forum threads, and social media posts, as well as content links that come from other similar websites. It’s necessary to find out whether these links are marked as follow or nofollow.
A rule of thumb that I follow is when I purchase a domain name I have to believe that I can flip it for a 100% profit.  So if I spend $500 on a domain I need to feel like I can sell it for no less than $1,000.  This way, even if I’m wrong in my estimation I still have some room to still turn a profit.  And if worse case comes to worse case I’ve sold domains before for a $0 profit (it happens even to the best of us).
I remember the glory days of the Internet where people did make a ton of money buying domain names. Until I came across this post I actually didn't even realize that people actually still thought they could.make.a.lot of money by doing this. I have some domains that people would be interested in buying and I get contacted about them but mostly it is for people wanting to purchase them for like $1200. And for that price I would rather just hang on to the domain name. I also get contacted by a lot of people offering to sell me complimentary domain names at what I would call ridiculous premiums. The knew google is all about branding. I would consider paying for a domain name that has a great brand attached to it but I could care less about most keyword rich domain names. Of course there are always exceptions.
1) pending delete auctions.  These are domains that were not renewed and are going through a pretty complicated drop process.  You can backorder these names at the auction venues and then each attempts to "catch" the dropping domain.  If one is successful you win the domain at $59 if you're the only one to backorder the name.  If more than one person backordered the domain a 3 day private auction is held.  Once the domain goes through the drop process, the back links have pretty much zero SEO benifit to the new domain owner.
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. 
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