I've thought about it several times. The issue isn't really writing the code. It's the bank backing part. Most banks won't go near that industry. So finding a bank that will actually handle the transactions would be the first step. Writing the software to add in a pleasant wrapper for all these broken services would be easy (ish), but time consuming. But yeah, I think it would be a serious money maker. Even at 5% per transaction in a multi-billion dollar industry, that's still hundreds of millions of dollars.
Can't you simply sell them something different instead so they never get charged for porn in the first place?
That sounds like a good idea; just handle the cascading yourself. Couldn't you sell access to - say - some music site or sell bitcoins and - as a kind of "bonus" - give access to the porn site itself? Just hide the main product behind something innocent like a premium account or something that won't raise eyebrowses from spouses.
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