If you are researching economics, then Marx is a secondary (or higher order) source. The econmomic data used by Marx is primary.
If you are researching Marx the person or Marx's ideas, then [edit: Marx's writings would be primary sources, and then] what would you think of someone who only read others talking about Marx, but didn't read Marx's actual writings? It wouldn't make much sense for anything called 'research'.
Well, that's how actual research is done - that's what people do. It defines research, it's how they write all those (research-based) books and journal articles. That's scholarship - science, social science, humanities, etc. That's why Darwin travelled all the way to the Galapagos Islands - he identified a place to do primary resesarch on the questions he was investigating.
Journalists also - they look at the actual evidence and turn it into something coherent and cohesive for others to understand; the location in the byline/dateline shows where they went - that is, that if they wrote about Odessa, the dateline should show if they actually went to Odessa and saw it for themself.
Back to research: If you mean it's impractical for many people, that's often true. Some topics require less expertise, expense, and time: for example, tracking local bird populations. But those real natural scientists, the amateur researchers who get published, that's what they are doing. Pretty amazing!
If you don't want to do it, that's fine; few do. If you want to read secondary sources on a topic - wonderful (but read good ones based on primary sources; only accept what is tied back to actual, hard-won facts - necessary are the footnotes, which connect the secondary claim to the primary fact). If you want to read tertiary sources, find quality ones. If you want to read social media, have fun but it's just entertainment - to the extent it's accurate, you can't know unless you already have expertise.
If you are researching Marx the person or Marx's ideas, then [edit: Marx's writings would be primary sources, and then] what would you think of someone who only read others talking about Marx, but didn't read Marx's actual writings? It wouldn't make much sense for anything called 'research'.