Generally, whenever there's confusion between the generic and proper versions of a noun an article is used to distinguish between them. No article refers to the specific (which might have nothing to do with the generic - ie "Apple" vs. "apple"), "a" refers to the generic, and "the" refers to a specific instance of the generic which, if capitalized, indicates a widely understood (as opposed to contextually understood) reference. If a noun can take on all three forms, it's preferred that the specific-instance-of-the-genetic form not be conflated with the specific-but-not-as-an-instance-of-the-generic form.
"Commercium," which by the way has an existing, albeit non-standard, usage: (
http://www.reference.com/search?r=13&q=Commercium), is a mating of "commerce" and "ium," a suffix sometimes used to designate an enclosed facility (ie Pandemonium, Coliseum, auditorium, atrium, castilium, sanitorium), which then roughly translates to "an enclosed facility for doing commerce." This image, conjured by the word, is notably generic, because there are always multiple facilities for doing commerce. It's easy to imagine "a commercium" where merchants might gather, as opposed to "the Commercium" that stands for the proper name of an organization or place. The result is that an article is needed to distinguish between the generic and proper forms of this noun (which you wouldn't need if there was no generic form - ie Ebay), even though it's an imaginary noun where the generic form is never mentioned - simply because of the construction and etymology of the word. Complicated, I know, but what's amazing is that common speakers of English who've never come across Commercium immediately notices that something is off - such are the subtle, unspoken nuances of language that escape the rules of grammar.
For juxtaposition, notice that Pandemonium does not require an article, because it's hard to imagine a generic place where all demons gather, and because the word simply sounds better as a proper noun. As with everything beyond grammar, there are no hard and fast rules - only guidances.