Legal Question in Civil Litigation in California

You do not describe a situation in which you have any lien right to the trailer. You may or may not yet have a right to dispose of the trailer, by sale or otherwise.

It seems pretty clear you have a bailment of the trailer, which is an old legal term for having possession of someone else's property that you have not stolen and that they have not given, sold, or rented to you or given you any other right to. Whether a bailment is voluntary or involuntary, the nature of the property, and any express or implied expectations of the bailor (the owner) and the bailee (you), all play a role in the law of how and when you may dispose of the property.

So without a lot more information there is nothing we can tell you about whether you have a right to sell it and/or how you would go about acquiring that right.

As Mr. McCormick points out, you have not provided much information. You can obtain information about DMV lien sales here: http://dmv.ca.gov/faq/faqliensale.htm