Very true about some suppliers not wanting to deal with you without a bricks and mortar store, that's about protecting the value of their brand. Brand name suppliers do not want their products turning on on an eBay being sold at a discounted rate over a bricks and mortar store because that devalues the products and then makes them hard to sell through the bricks and mortar marketplace.
It creates an environment where the offline sales seem far more expensive to the online, then you have a slowing of offline sales and that can threaten the entire value and distribution of the brand, and that's the last thing a brand name supplier wants.
I like your idea of having items commissioned for your own branding, that shouldn't be an issue at all as to where you sell them as they are your works to sell legally. That said, if you did do that then I would suggest developing your own site, branding it to the image you want your business to present and then using sites such as eBay to drip feed loss leaders through to drive traffic to your site. So using your site as a the money making tool and using sites such as eBay as nothing more than a marketing tool.
Nothing wrong with a 6 week jewellery course lol, Necessity is the mother of invention, but invention is the long lost cousin to success. Link up the family tree, give it a shake and who knows what opportunities will fall from it