Selling can be presented as your employment, as in selling to others in a retail environment; or it can be more personal, as in selling your possessions. Obviously, if your waking livelihood includes sales, dreams of selling will not be unusual. Unless you sell something particularly sentimental, most of the remarks here will not apply to you.
If you are employed in selling in dreamland, the emotional transaction at work is persuasion. The object you are trying to sell may be ridiculous or an actual consumer item. Depending on which it is, this may reflect whether or not you feel you will ever be persuasive. Your clientele also is important to this dream. Do they have something in common?
Do you see yourself as persuasive in waking life, or do you need to develop those skills more fully? If you are having a yard sale in your dream, you may be trying to creatively solve a financial problem or be feeling burdened by your possessions. If the yard sale has an element of emotional darkness, there may be financial anxiety involved that precipitated the need for downsizing the household. Dreams of this nature often try to show feelings of over extension. Heed them.
If you are selling items of great sentimental value, you may be seeing yourself as divesting from a particular relationship. This can happen at times of transition into adulthood or (desire for) marital separation. Many times, you do not actually perceive the emotional content of the items in your dream the way you would in waking. They are simply "extras" that you no longer wish to carry through life.
The act of selling in a dream, usually, is in the most general terms symbolic of some sort of emotional transaction that may be taking place in life.