It is vastly easier to BUY domains with optimistic goals of re-selling same then it is to actually SELL the newly acquired domains for profit.
This is a key element, and prospective investors should read it over a few times, and let it sink in.
It's easy to buy quality names. Recently I was looking to start an online business, and there were lots of expired-but-not-yet-registered domains that were very business-worthy for my purposes, but there is no guarantee I would ever be able to find a buyer for them. Finding viable names is easy, but reselling them for a nice profit is very difficult.
Most domain action at the upper end revolves around upgrades, like MonitorSystems.com moving to Monitor.com or Twist.io shifting to Twist.com. That makes it very difficult to access that high-dollar market, as the vast majority of these premium "upgrade-ready" domains are either in the hands of veteran domainers or owned by multinational corporations like Google or Amazon.
Even at the lower end, it's tough to determine what one buyer will think is gold or trash. I see a lot of sales data from the various domain sales sites, and some of it makes no sense, like a keyword-stuffed domain with 3 hyphens selling for $5K? Maybe the guy thinks it's still 1998? Other times I see a nice single-word .COM going for $3K and wonder why it went so low. Did the owner need the money? Probably. Often there is no logic behind the marketplace, and beauty is always in the eye of the beholder. And if you look close enough and ignore the outliers, you can see the trends.
There are ways to succeed, and one of them is having a plan and sticking to it, hell or high water. Domaining is not about the fast (or short) money, it's a slow trip up the hill, but if you have the patience and ability, you just might get to the top. Find your niche, renew and drop with skill, all the while consistently upgrading your portfolio.