Considerations

Here are some things to consider when running an e-commerce site:

Knowledge of html and css: Even well known shopping carts require tweaking and some knowledge if something doesn't look right or where to look if something goes wrong from a design standpoint. Therefore, it would also be necessary to have a basic understanding of the directory structure.
Graphics: Maybe you'll need a logo or a picture modified, there is always some need for having graphics skills, sooner or later.
Administration: Whether you host your own shopping cart or hosting with a pre-configured one, you'll need to know the terminology and all the different options of the control panel.
Products: Are you paying per product to list them? Or another option? You'll need information and pictures for your products. Updating your products can be time consuming, so you want to do it right the first time. If you figure each product can take up to 5 minutes when you first post it and maybe a couple minutes when you update a product, you can see how time consuming this can be, especially when you get into the thousands of products.
Shipping: How are you going to ship your products and how will you interface those shipping providers with the shopping cart?
Taxes: You'll need to know a little about taxes and how to charge taxes to the correct people and where to set that up in the shopping cart.
Promotion: You'll need to get your website into the search engines. Some search engines like google will find your site eventually if you don't register, but it will take longer.
Webmaster Tools: Google and other search engines have tools to specify a site map or find crawler errors, to name a few functions. It's in your best interest to know these.
Directory Listings: Whether it's yahoo, yelp or the yellow pages, etc. You'll want to list your business in as many places as possible.
Keyword tools: Whether you're doing advertising or not, you'll want to know what keywords are important for your customer base and how to translate the proper keywords into interesting content on your website.
Advertising: It's always helpful and doesn't need to be that costly, but you'll need to learn a whole new set of terminology and all the options in the advertising control panel. You'll also need to know how to be restrictive so advertising providers don't wipe out your whole budget on the wrong viewers.
Statistics: Without looking at statistics, you have no idea how many people are going to your website and who referred those people. If you're paying for advertising, you'll want to review the advertising statistics to know your money is not being wasted.
Security: It's a real headache these days. Even if you're not handling the security, you're credit card provider may want answers, among other things.
Vulnerabilities: Even the best shopping carts are subject to security holes. You'll need to be aware of that if hosting your own cart, and even if you're not, it's a good idea to be informed about your shopping cart and environment it exists on, especially if you're managing it.

This is by no means a complete list. It's just the first things that come to mind while writing this and also only gives bullet points to the most easy method for opening an e-commerce site. If you're planning on running your own server, site, and security, the list becomes exponentially larger. New challenges are presented monthly, if not sooner. That's what makes a service like MWV quite unique and incredible. We do most the work with no out of pocket up front. For the areas we are not subject matter experts (SMEs), we will assist you to get that subject matter content where it needs to be.