Hey markec123, where are you living?
I'm currently doing the same in my country, offering small businesses to sell their products on my website. It's a city focused site so I think it helps to narrow your efforts and to make people feel more approachable to you.
Just call some of them and offer publishing their products for free for one month, so when you launch you will have something to show. Try to figure out the shipping and selling process. What about buying some books about Sales and trying to apply that knowledge inmediately?
You can use Woothemes to start and its ecommerce plugin for WordPress.