The easiest way is to go  Selling  >  Sell.  Here you have two options   -

1. Find your item by searching for it using its Name, Brand, Series, Reference #, UPC or more.
   
   
2. Skip that process to create a Generic Listing by using this button  -  

In both cases fill out the form.  Your item will get more exposure if you assign it to a Database Item as about half the sales on our Sites occur via the Database Items page.