The auction is here. This is not my item and apologies if this particular one has been seen before, but it is one of the cooler custom trailers that I have seen. The style goes very well with that of movie Optimus. It is scaled for the Leader Class figure, and even opens up to reveal a flat bed with several guns. An additional thing to do to it would be to take Scout class Hardtop and make him into a custom Roller.
It's nice, but just having a flat trailer with a pop-top and a bunch of guns on it doesn't exactly fit the movie "feel".