What constitutes a "module" on here is somewhat different from a traditional module that you'd get in print or PDF. On Roll20 a module is a collection of In-Roll20-resources set-up nicely to help the GM & players, along with any adventure text, outline, characters, details and such. I think it's actually meant to be more like a "Roll20 Campaign" that's pre-set-up. I'd recommend purchasing the modules that are currently available here (Not very many so far), playtest them with your group, then design your outline of what resources you want to produce to make your module something that would appeal on Roll20 Marketplace compared to the existing offerings. Note that if you have text-only write-ups that you want to publish/offer as adventures without graphics, you could do that on another website (such as Lulu for offering Print-On-Demand or PDF downloads) and then your buyers/readers could run that adventure on Roll20 or anyplace they wanted. So if you have a module creation style and skillset that doesn't fit the Roll20 Marketplace format, look into other methods of self-publishing.
