This page will explain how to use the different types of vendors. Generally it's just right click -> buy.
Its usefulness will be in troubleshooting -- explaining what to do if you don't receive your product, and how much of a normal thing that is.
A complicated object where you click things and arrows to change what's displaying. After it's displaying what you want, you can right click -> buy.
Sellable Box or Object
A non-scripted box or object. Just right click -> buy and it will give you a copy of the object, or the contents of the object.
Finding Bought Objects in Inventory
WAIT this will just be a quick blurb. A link to a more complete tutorial is in "Opening Boxes"
Where things appear in inventory, and how to find them. The chat or blue popup tells you the name of it. Inventory can be sorted by date or name. It will be a folder or an object. Objects go into their respective directories, and folders go into the root.
Note on Inventory Management
Keep things orderly. Don't store everything in the root. You can store avatars in subfolder in the defaut "Clothing" folder, or create a new folder like I did and named "Avatars."
Linden Labs provides chapter six of their Second Life book for free on how to manage your inventory. http://secondlife.com/corporate/slguide.php