So you want to sell stuff on the Internet, and eBay and Craigslist just aren't doing it for you anymore. It's time for a real, live, grownup website of your own. But God, there's so much to think about. An eCommerce website is much more involved than a static website - it's always changing, it must have search capabilities, and you absolutely have to be able to accept payment securely.
First, calm down. There's a process for this. If you have the money, you can actually hire someone to take care of the whole thing for you - but smaller companies aren't left out of the game. You absolutely can do this yourself, but you have to commit to seeing it through.
Location, Location, Location
If you don't already have a website (why not?), you must first register your domain name and pay for hosting (unless you have your own server). A basic website is pretty easy to design using a basic web editor and templates. If you already have a site, plan how the shopping section will fit in with your existing pages.
You will need to choose a shopping cart software - some are simply plugins that work with pre-existing website templates, while others are entirely separate customizable things that require a much deeper level of involvement. Choose one that suits your business and your product. Look for things like search capabilities and display - the closer it is to what you want, the less work you have to do before you can just plug in your stuff and go.
The shopping cart software is what allows you to take orders, calculate tax/handling/shipping, process returns, send receipts and notifications - in short, it's your virtual sales clerk. There's no reason to blow your entire budget, but don't go cheaper than you have to.
You have to have the ability to accept payment online, and the customers must feel secure in the fact that their information is going only to you, not some phisher in a dark basement somewhere. A secure server is the first step, but you also need a safe way to process credit card information.
First, set up an Internet merchant account with your bank to allow the money to flow from the customer to your account. Then you need a payment gateway account, which acts as an intermediary to facilitate the transaction. Your shopping cart software may come with this capability, so double-check before spending money on another service.
In lieu of all that, you could also just enable PayPal on your site. People trust it, and it's secure. They don't charge a monthly fee, but they do take about 3% of every transaction, so that's something to think about. In fact, even if you do have separate payment processing, it's still a good idea to offer customers the PayPal option because many people won't trust any other form of online payment. This is especially true for smaller businesses that do not have a national reputation. PayPal also offers basic shopping cart software for a low price if you don't need any super-customized functionality.
Once you think your site is ready to go, run a few checks to make sure everything is working properly. Pictures should load quickly, pages should scroll smoothly, and the site as a whole should function quickly and properly. Send a few transactions through to make sure payment goes through right, shipping costs are in line, and auto-mail like receipts and shipping notifications are sent as scheduled.
Double-check spelling, photo orientation and size, verify that links are working, and run a load test to make sure the site can handle heavy traffic. Then go live and sell hard!