Hi vicking,
There is no limit to the amount of subcategories you can have, though I would imagine more than 20 or 30 might make it difficult to navigate around.
It is possible to mark items as digital and therefore no need to physically send them. The system allows the seller to send a link as part of the completion of digital products, such as where they would download an item from.
In version 2, which is in Beta stage, it is possible to 'sell' an item without cost, or offer it as an exchange.
Hope that helps.