Purchasing a large number of domain names as an investment, which you are looking to profit from, may not be ideal considering domain registration is not free and holding a portfolio of a few thousand domain names can be costly in the end. This does not mean it is not possible to invest in domain names. What you need is a keen sense of tech and industry trends that will help you make a smaller number of domain registrations that will be profitable.
Some domain extensions are better than others
Current trends do point to a small number of domain names that have a rise in popularity. Particularly .APP domain names are catering to application developers, .IO domain names have their audience with the tech startups and .AI domain names are the ones that aim deep-learning, machine learning, and artificial intelligence audience. If you are able to spot a popular niche domain name with one of these extensions you may be able to make a profit.
Arguably, .COM namespace is not available, but as new technologies and trends emerge, being first to register a two-word generic .COM domain name relevant to these new trends can lead to profit.
A lot of good domain names expire because the owner has forgotten to renew or he is shrinking his domain portfolio. Experienced domainer is always on the lookout for high-quality expired domain names. It so happens recently that a renown domain reseller has acquired an expired domain name for $50 and was able to resell it after a few months for a few thousand dollars. Not everyone can do this but expired premium domains are and investment category of their own may, and if you can market your domains, profit from a single domain transaction is much higher.
Marketing your domain names to potential buyers
Domain parking for domain names that have organic traffic can lead to small profit from ads that may cover the registration cost, but it may present your domain name in a generic and spammy image which may be overall negative demeanor and scare away visitors. This is why I would rather recommend that you invest into an affordable hosting and develop a small website that is relevant to the business or industry which your domain name is targeting, with a notice that will inform visitors the domain name is available for sale.
Even if you do not mean to sell your domain names for a small profit, listing them on popular domain marketplace websites can lead to better visibility for them. Take advantage of the free marketing these websites offer. Sometimes you will actually need to sell a few lower-quality domain names to cover the renewal cost for those you wish to keep. Again, Sedo or Flippa can help you unload some of these domain names and make the needed renewal money.
For some, selling domains is a good part-time business, for others, it is much more. It requires research, understanding the fundamentals, analysis, and marketing. Purchasing a domain name is worth your time in researching it before you buy, or sell. And while you might have a great domain(s), it might also require advertising to give it a push and to get noticed. But once you sell it, wow!