Like everyone has said pictures. All sides outside and views from inside. If you can find a picture of the floor plan that is nice as well. Be detailed in the title and make model and year. Also if you can add size of furnace, AC, fridge, propane tanks, and water tank sizes (fresh,gray, and black). If it has a generator add make and model along with a picture of the unit and how many hours. How old batteries and tires are is another nice thing. Can people look up a lot of that info on their own yes... but face it lots of people are lazy and if you can put that info in there it is easier for them.
Before you take pictures make sure its clean inside and out. Make sure all the trailer lights work (blinker, brakes). If someone does come to look at it be sure you can at least put power to the unit so that they can see that everything works.