Just a few thoughts...
If you can't get the product shipped, you won't make money anyways.
Open Source does not have to === not making money (you just have to think of a different business model than selling your code and/or a license to use it).
What is more important to you: the $$ or the experience? (IMO, experience is what you turn into $$$)
Just because your first project is open sourced, doesn't mean that your next project has to be! (or go contribute to someone else's project if it's the experience that you are after)
Is this functionality something you could sell as a module/package/upgrade for something that already exists in the marketplace so that you wouldn't have to create a whole ecommerce solution and could just focus on your idea instead?
There's already a TON of ecommerce solutions on the market (I'm also working on developing my own, have been for years - but for me to build my own products/services, not for others to use themselves). What would inspire me to pay to use yours over something that's already on the market, and possibly also free? (not saying that competition is a reason to not do this - I view competition as validation!)