Wikipedia can be a great resource for a lot of topics. But no matter how many good articles you find, it will always lack one thing: authority.
All articles on Wikipedia are unsigned, so you don't know who wrote it. And while new material does get reviewed by Wikipedia editors, this is not the same thing as the peer-review process that takes place in scholarly community for resources such as the Stanford Encyclopedia of Philosophy. So unless you come to the article with extensive knowledge that allows you to critically evaluate it, you are at a disadvantage.
Stephen Colbert has commented on Wikipedia a number of times, the most infamous of which was his effort to save African elephants by insisting that their numbers had tripled. Soon afterward, Wikipedia had to lock down the article to prevent Colbert Nation followers from changing the article. Jimmy Wales later came on the program to tell Stephen how much he appreciated his efforts.
One thing is true of all encyclopedias - they should always be a starting place for research, not the endpoint. That's why it's so seldom that one sees encyclopedias cited in the scholarly literature.
These dictionaries can clarify terms used by Marx.