Ditto that it depends on your state.
In Florida, anything over 3000lbs has to have a title before it can be registered and have plates. I was not aware of that so when I bought my trailer used and was given the MCO, I thought everything was golden. Until I went to the DMV and they told me that this trailer had never been registered with the state of Fl and the tags on it were for a utility trailer.
Long story short, the guy who bought the trailer new bought it out of state. That way he didnt have to pay sales tax. He then proceeded to NOT register it (as he would have had to pay taxes on the sale price then) and put illegal tags on it.
In order for me to have it titled and registered in FL I would have to have the BOS from the trailer company to the guy I bought it from, proof that he paid his sales tax, then all sorts of fees to put the trailer in his name and then transfer to me, etc etc etc. Florida makes you bend over to get anything done and they make sure they get their money twice over!
I have since found out that I can get the trailer titled and tagged in the state of Maine even without living there due to their laws. It is VERY common for companies to have their trailers registered in Maine (think semis, etc) as its so much cheaper. I can get my trailer registered titled and tagged for $165 for 5 years. Totally legal and legit. (unlike the guy I bought the trailer from).
IF your state requires a title and you dont have one, I would look into this company (http://staabagency.com/ ). I have spoken to a lot of people who have used this company for business and personal use and they are totally legit.