Licensing and approvals are a bitch.
I worked for a company that was heavily involved with that, although we were manufacturing Halloween costumes for mass market retail. You would have to negotiate a separate licensing agreement with every copyright holder/artist. Even if you had your own attorney (you do have your own Stripcreator general counsel, right?) draft a master agreement, each would be unique because different sets of lawyers would make different changes and you would incur thousands of dollars in legal fees just to have all the agreements lined up. I realize most of the entities involved aren't gigantic entertainment corporations like I dealt with, but that would only make it worse since large corporations tend to use the same boiler plate language and are inflexible about changing it.
There would also be a matter of royalty rates and guarantees. Trust me--you'd never sell enough copies to pay off everyone (especially if your only distribution is Cafe Press).
There are other intellectual property issues, but I think I said enough.
