I see several drawbacks to a store like this
1. The over all cost -- Assuming the lowest of 100 products your year 1 cost is $350.00 and $300.00 per year thereafter-- with a site of your own about the same size you would have a $700.00-$1200.00 initial design cost (to have someone do it for you) and about $50.00 a year max for hosting and domain name.
2. The domain name is somewhat a problem too
http://YOUR STORE NAME.mybigcommerce.com/ will tend to put some customers off Unless your familiar with domain masking (which makes site verification with google etc a pure bear) My personal opinion Get a designer (like me maybe LOL) to set your site up configure all the addons like the cart etc input the logo and hand you the administration panel to input products That site should have a 1 time cost under $400.00 and whatever your hosting/domain costs are ongoing