Difficult to answer without seeing the full picture -- like, are we talking annual sales of $5,000, or $100,000?  High profit margin, or low?  And just because they don't want to call it a partnership, doesn't mean that you don't have one.  

At any rate, you have a conflict of interest if you start giving advice to the ex.  That marital partnership has already dissolved.  You don't want to be subpoenaed when the fight over trying to fool eBay starts.