Trailers are a high depreciation item. They make the depreciation on a Hyundai Excel look reasonable.
If the trailer is more than ten years old it not only isn't likely to have much value, but may be a detriment to the land value of the parcel it sits on. If you really want to buy a double wide, buy a repo, a very new repo can be had dirt cheap. In 2003, when I was looking for a place to buy cash (found what I was looking for dirt cheap by the way), I looked into buying a repo double wide and moving it onto some land I have out in the sticks. I was looking at a very nice two year old double wide, delivered, set up, and everything for $15,000.
As for trailers over 10 years old being valueless, what I am saying is that typically the value of the trailer is less than what it would cost to move it and set it up elsewhere. If buying land that has an older trailer on it be careful, the trailer could well be a detriment. Older trailers with little or no value can be hard to get rid of, often the only way to do it is to hire somebody to tear them down and haul everything away.
I have nothing against trailers, but these are the facts of life with mobile homes. I know lots of people who get suckered into insane ripoffs for trailers, they will get talked into buying a 70K double wide, with all the bells and whistles, and then another 20K for a half acre of land that isn't worth 5K. The really sad thing is that in this area they could have bought an existing stick built home for way less, and in ten years it likely would have appreciated.
Of course there are exceptions to the rule, but that doesn't mean you can count on being the exception. My mom's neighborhood is on Strom Thurmond lake, outside of Augusta. Back in the fifties the Corp of engineers sold one acre lots for $500 lake front, $300 on the other side of the road. My grandfather bought two acres lake front, built a house for about 3k, the house has been well cared for since, my grandparents died in the eighties and my parents live there now, the property is assessed for taxes at $350K, although in reality, even with this shitty economy their land would sell almost immediately at the bargain price of $750K, a lot similar to theres, but not as nice sold for 800K last month, after only about two weeks on the market. The doctors and lawyers might be having a tough time, but they aren't completely broke, and land in that neighborhood is in high demand. Way back when the area was sold of by the Corps, one of the lots was bought by a man who just wanted a spot for a fishing cabin, and that is what he built, sometime in the late seventies he put a small single wide trailer on the land. A couple of years ago he sold the acre and trailer for nearly a million dollars, but that really doesn't mean that the buyer thought the trailer was worth that much.