The word broker means a salesman who controls a product for sale. Incorrect usage cannot change the definition.
Incorrect usage most certainly can, over time, change the definition. To see just a few examples, have a look at the following site:
http://m.neatorama.com/2009/06/29/words-that-changed-their-meanings/
Here’s an article all about a single word, but it’s crystal clear: after decades of misuse, the word literally took on a different meaning.
http://www.theguardian.com/commentisfree/2013/aug/13/literally-broken-english-language-definition