Make sure when posting what you have to sell, post images.
A OMM is a offical middleman, people who have earned there trust with thousands of dollars and basically make a safe and easy trade for you.
You use a middleman for IRL objects in two ways, the person shipping the objects will send the product to a person, person will confirm he got the object and will tell the MM. Or, the persons shipping will ship it to the MM and the MM will give him the money, and therefore shipping the item to the correct person.
Any other questions, PM me.
Regards, Tyler.