Domain names expire when someone decides to stop renewing it. They may not be available to register immediately. We update the search index every night, so some names may already be renewed or re-registered. Some may become available in a few days. Some registrars, like GoDaddy, will let you buy a name that one of their own customers expire immediately. When you find a name you like, just click on it to try backordering the name!
The "Domain Lists" and the "TLDs" filter are now static and not created dynamic based on the current search. Turns out creating these 2 filters dynamically was slowing down the search tremendously. It even made certain requests impossible. For example now you can do a dot search that affects all .com domains and get a result with over 150mio domains in reasonable time. I'm also working on a new search cluster that will make searching even faster (not released yet).
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. 
At the end of the day, the owner of this domain name let it expire and didn’t renew it. So while there may be many legit reasons as to why they might’ve done that, one of the not-very-legit reasons could be that the domain is “dead” in some way or another. In other words, it’s no longer useful for the original owner because of a penalty or otherwise. If so, it's unlikely it'll be of any use for you as well.
If you don’t have a lot of money to invest in employees and you’d rather work on your domain portfolio by yourself, you’d need plenty of free time. Generally, mini sites with 5 to 10 pages of content can be built within 5 days, while authority sites with over 10 pages could take up to two weeks. This amount of time would depend on your writing skills and knowledge of SEO techniques.

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HIM: "Mainly small-medium sized local businesses looking for a way to increase their ranking. Most already have a branded domain in place, and have heard that using a keyword domain can help. Or they have seen a competitor ranking using that method. The problem I face is that they don't have a lot of money to spend, so I get lowballed on the asking price. There have been a few that make a ton of profit vs what I purchased it for, but that boils down to luck: what is for sale, when I find it, things like that."
Expired domains are domains that have been registered by individuals, businesses, or organizations, but aren’t renewed after the contract ends, or are deliberately terminated. That means that they available for re-registration. There are many reasons why someone would choose to abandon a domain; one reason could be that the web project failed, or that a domain portfolio went into liquidization. Domains can also end up becoming free due to the owner receiving warnings for possible trademark infringement. Re-registering can also result in legal consequences.
Here's an old post on search engine roundtable that claims google's policy is to discount previous backlink juice when a domain changes ownership.  I'm not convinced whether this is actually true or something Google says to discourage excessive domain buying / 301 redirecting for SEO benefit.  The comments above seem to give varying opinions on this matter. Would be great to get to the bottom of that one!
Your course of action really depends on how much work and effort you can put into the expired domain. If you're barely able to maintain and optimize your current site, you probably want to just 301 redirect the old site (note: see my amended comment above about the link value not likely to be passed). If, however, you're more creative and have some time on your hands, you can try your hand at crafting a microsite. If you really know your stuff and are experienced at making money off various websites, you'd probably do well with the third option. 

If you are looking to monetize an idle domain, looking to third-party platforms would be a good idea. Many of these platforms such as Nameforest ourselves will create a logo alongside your domain, which will be added to the list of preexisting domains on their sites. This is useful as the demand for your domain will also be greatly increased after it is rented as opposed to a domain that leads to a blank site - thus making it easier for you to find a potential buyer.
HostGator often offers promotions, coupons and special offers to customers during their initial term. Please note that special offers are limited-time promotional prices that are available to new customers and are valid for the Initial Term only, and not for successive or renewal periods. Promotional rates apply to GATOR, Shared, Cloud, VPS, Dedicated, WordPress and Reseller hosting plans and will automatically renew after initial term at regular rate found in your control panel. Note: If you register a free domain through us and wish to cancel your account, there is a fee to retain your domain.
They see a domain’s price tag and they can instantly tell whether it’s under or overvalued. This is probably the most important skill one should have in this industry. Neil Patel has an interesting related quote: “The most important thing to remember when buying sites is that you always make money on the buy, never the sell.” In order to successfully do this, you must be able to spot undervalued domains. This is what will ultimately translate for you into a handsome profit margin when you get the opportunity to resell the domain for its “real value”.