I have one domain name which I never used it and it has .com and I later discovered there is another company with similar name but with .net and they are doing good as once they approached me to sell them my .com which I never did as I had plan to start something which didn’t materialize . its been 11 years now I have maintained the name now I have two options.
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.
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.
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.
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.
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.
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”.