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"
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.
If your domain name is related to a particular niche, then you may find success by advertising in forums that are related to that niche. Simply placing a link in the signature of your posts can tremendously speed up the selling process. While it may be possible to find prospective webmasters on niche-specific forums, it is generally better to advertise on webmaster forums, where there is a high volume of potential customers.
Hey, what’s up everybody. It’s Brian Dean from Quick Sprout. In this video, I’m going to show you how to buy affordable expired domains and how to evaluate the potential value that an expired domain could bring to your site. Now, expired domains is pretty gray hat, black hat type of thing because there’s basically two things you’re going to do in an expired domain.

Right now there are over 656,000 expiring domains that you can pre-bid on at Namejet. Lots of opportunity there even if more people come. The key is to find those hidden gems and wait until 1 minute before they go off the board (assuming no one has bid on it yet) and then bid. This is the best way to attempt to be the only bidder on a domain and win it for $69.
Also note the Expired Domain Plugin is a lifetime “per user” license, so if you have multiple ScrapeBox licenses registered to the one email address purchasing the Expired Domain once will activate the plugin for all your ScrapeBox licenses with the same email for life. The plugin is a one-time payment and is not a monthly or yearly subscription, all updates are free.

However, I'd argue that most people in the business really struggle as they don't understand what makes a good domain name and what's a bad domain name. While I do not work in this field full-time, I can say that I can make a living of it, but it's probably because I don't ask for absurd prices (e.g. randomword247.com domain name for $8000 is just unrealistic and won't happen) and therefore can sell domains easier. 
It is mind boggling how local search engine optimization has grown from a tiny market to a mammoth industry within a span of ten years or so. This is also one of the reasons why focusing majorly on local names is profitable. From doctors to pizza parlors to salons, everyone wants to see their business thrive on the first page of Google Search results with the help of target keywords.
Buying domains or registering domains–what’s the difference? There are many ways to acquire a domain. While available domains can be registered with any domain provider, rights of use from already-registered domains have to be purchased from their current owners. You can find different market places and auction platforms online that are tailored to these purposes. This kind of domain trade is...

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.
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.
Domains that are indexed on Google and highly ranked are particularly attractive to traders. Other SEO aspects, such as backlink profiles or the search volume of the keywords in domain names, also play a significant role in appraising value. Design can also positively affect the price of a domain. Short and succinct names that are easy to remember are especially advantageous. Endings are further factors that should be taken into account. Top-level domains (TLDs) such as .com or .org are by far the most sought-after endings.
Please keep in mind that domain flipping isn’t child’s play. As much as you may want, it isn’t easy to buy and sell domain names for huge profits without putting in a good amount of time and effort. Although there’s nothing wrong in aspiring for at least one big deal per month, be aware that sometimes it takes months and months to sell just a single domain name.
Above you provided great information about buying and selling domain names. I have been to many blogs about the same and I have read the people make millions just by selling domain names. It's a process that one needs to learn with time and can give a good earning by reselling the domain name. The business is fruitful but one needs to read, understand the value of a domain name. Buying every domain name is a waste of money. One must see the future potential growth for that particular domain.

The easiest and least time consuming option is to 301 redirect the old domain to your existing site. This tactic obviously works best if both sites are in the same sector and are targeting the same keywords; otherwise, if you have a pet supply site and you buy an old Texas Hold 'Em poker site, a redirect probably might raise some eyebrows among the search engines. If, however, your site is brandnamepets.com and you buy onlinepetsupply.com and 301 redirect the domain over, you're inheriting a lot of topical and appropriate links.

What if we replace “bitcoin” with “ripple” in all these examples? SellRipple.com? RippleGiftCards.com? Ripple.network? Ripple.us…etc. You get the picture. There are a 100 of these on NameBio + the keywords we got from the Google Keyword Planner, that’s hundreds of potential high-value domains right there. The thing is, we’re only scratching the surface here. According to Wikipedia, there are over 1,300 cryptocurrencies online as of January 2018.