Keyword relevancy is not the only thing that matters when it comes to SEO. However, domain names that were previously used by other businesses usually have a lot of backlinks and a decent ranking on PageRank. Even though such domain names are usually about to expire, they are still worth a lot of money if they have a good number of backlinks and a ranking of five or more.
I have an interesting twist on how I have done it in the past. I will buy a domain and use it as a development domain for blog posts and things of that nature. Then over time it will gain DA and PR. Then I will sell it off and buy another development domain and start the process over. Not quite the process you mentioned, but it works. People will link to the development site to show things off as well sometimes, so that helps.
There is a misconception that good ‘.COM’ domain names are hard to find and so they are more expensive to buy. But once again, this statement holds no water. You can easily check out the ‘.COM’ domain names that are on sale at highly affordable prices. However, the only downside to flipping ‘.COM’ domain names is that you will have to discover them at different places like Flippa.com or private seller portfolios.
Your domain name registrar may provide you with a free one-page Website tool, which you can use to create a “this domain is for sale” landing page. Alternatively, you can create a single page (perhaps a hidden page hanging off one of your existing websites) that indicates that your domain names are for sale. You can then forward all of your domain names that are for sale to that one page.
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.
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.
So how do you develop that instinct? NameBio maintains a database of over 500,000 historical domain sales (as of writing this post). They have interesting filtering features by which you can narrow down domains by price range, date sold, keywords and more. Simply sifting through the listings on NameBio long enough will quickly develop your domain appraisal “instinct”.