How Do I Sell Things?
To sell things you have to own them. Things that you make belong to you and you alone. You have the creative rights to that item and no matter how many you give away or sell, it is copywritten by you.
Now with that said, this only means that someone cannot take your design and make it theirs. Just as in Real Life, you can buy and sell your property at any time, but as with Real Life, there are laws that protect a person from copywrite infringement. If you're interested, please visit Second Life or follow this link to the dmca for more information.
If you own an item. Say it's an oriental rug, and you no longer want it. You can put it up for sale in a yard sale, just as long as the creator gave those rights to the object. (Next owner can sell, give away). You just sell the item as the creator of the object.
Another way is to rent vending space. Many new citizens with great ideas will start off this way. The space can be quite small, but very effective if it's in a mall. Yes, a mall. Just like Real Life where you have anchor stores, with dozens of smaller merchants, Second Life has malls too. Malls attract people and the Anchor stores are what brings people to them. A small vending space in a mall can make you quite a bit of $L's.
Still, if you have the merchandise to sell and you don't want to pay a landlord rent or don't want to be part of the corporate american chain of shopping centers, you can opt to purchase land (for pay accounts only) and build a store to your desire. Owning your own store can be fun, but very time consuming. New items need to be stocked. You have to monitor your prim limits, the type of vendors you want to use. Do you want to be independant or on a network like JVEN? It's all up to you though. You're the boss. And all the customer has to do, is pay your vendor.
Except for - in most cases - yard sales, if you rent or own, you will need a vendor. Vendors are normally single scripted prims that are set on walls. In SL, you can "pack away" items into a box, cylinder, orb or any other shape of prim. Dragging an item and placing it inside a single prim is a great thing, because you can have a limitless number of items in a single prim (packed away), but it still only counts as one prim. Remember though, no one wants to stand an wait for an image to rez, because you can only see images of one item at a time if it's in a vendor that holds multiple items. It all depends on what you want. A single vendor for a single item or a clickable vendor that will cycle through images of your goods for sale.
Here is a photo example of what a vendor might look like. Click on it to see a larger image.