I think a "module" would be a file containing of maps, tokens, handouts, and their associated data. To make this convenient, you'd need to be able to save, share, and install the 'module file' as a single item. Kind of like a small "campaign" that you can zip up and transfer (i.e., "sell"?) to another Roll20 user for them to run.
Unless Roll20 already supports this, and I missed it. Wouldn't be the first time :)