To Felixdacat,
You need to focus on your startup, and learn business as you go along. Bschool will be an option for you later on after the experience of doing this on your own. At which time, MBA programs will actually want you to apply. And in the end, you may not want to, especially with upside potential for blockbuster success. I'm glad to have found a useful topic on these boards, considering that there's a kid with a "High School" mentality (who's doing an undergrad at NYU, with no work experience or aspirations on your level) trolling on these threads, giving advice about which bschools to attend at the grad level (what's scary is that other posters are actually listening to him). The key thing in your post is that you're looking at the situation of MBA vs. Startup from its proper perspective.
You won't be able to do both, even though the online retail concept has significantly lower operating costs than beginning with an offline storefront, for essentially the same items. But there's a whole different type and set of marketing, operational, and financial challenges related to systemization and scale, coming out of the gate. Positioning it as a side job multiplies the chance of failure, as success in this situation can only occur if this is what you're committed to doing (you should burn the bridges, and not give yourself an excuse for failure). You know that markets in urban design are highly fickle, and become more fluid in their unpredictability, over time. Perhaps, you can use your academic background to establish a portfolio-based communications strategy to bring out target buyers/site users through highly integrated, but cost-effective media. Then you can create and exploit off and online market feedback loops to correctly forecast and select design styles for upcoming seasons and events (let the market design the product). And if your selection can narrow optimally (Yes, treat it as an optimization problem), then you can save on scaled volume purchases with discounts of original urban designs. You don't need an MBA to do this, but the experience of handling market uncertainty through strategic adaptation and growth through tactically applied "will" is crucial.
Network with suppliers, designers, investment sources, etc., and build your resume through your own startup experience. Even if you decide to go to bschool later on, a letter of reference from a business source in your supply chain or a potential investor you've periodically negotiated with (or better yet invested in you) is far more impressive than one from a direct supervisor at a large company. You can obtain what you're seeking through your own efforts in building your startup. Remember, MBA = Option, while Startup = Undertaken Risk in the pursuit of upside and blockbuster success.