Well, I wouldn't try to categorize it, what I would do is describe in detail what has already been done on it, how useable it is (ie., could you go camping this weekend, or is there critical stuff to be done), and how "modernized" it is (ie., does it have grey tanks retrofitted, etc.).
So, it is hard for any of us to look at the pictures and tell you what it is worth without knowing more information. A trailer can be "half renovated," but if the axles are original (ie., shot), the floor has rotten spots in it, and there are major appliances that are not "RV" quality (ie., a dorm refrigerator rather than an RV frig), then all that interior paint and upholstery aren't worth very much.
So you ask "what should I do before selling?" It depends on the level of useability you intend to advertise it as. If you are going to tell me that it is "camping ready," then it had better be ready to go boon-docking. If you intend to call it "safe to tow, but a work in progress," then it probably doesn't matter that you do anything more to it, as long as it is priced appropriately. At a glance, though, I am inclined to say that you should treat it like you would a house: A little bit of interior paint and caulk can really make a house more sellable. I've walked away from houses with bubble-gum green and purple rooms, simply because I don't want to start my ownership of the house by painting over what somebody thought was cool. Many, Many people who buy vintage trailers do so planing to "make it their own," so no matter what color you paint it inside, the new buyer may have other ideas.
So back to price. People can be convinced that the "heavy lifting" is worth money--repaired rotten floors, replaced major appliances, fixed leaks, repaired rusting frame, etc.. When it comes to the interior paint and upholstery, you have to find a buyer with your exact same tastes, who can look at the work you have done, and think "great, I won't have to do that."