Despite all the scams going around on Ebay, there are several honest sellers out there.  What you have to be careful of is the condition of a used trailer.  If one has ever leaked, there could or will be more dammage than meets the eye, the older the trailer, the worse it can be.  When a leak in the roof happens, it soaks the insulation and wood, it can drain down the walls, spread to the floor and do the same thing.   After a period of time, it will spread.  The wood in most of these trailers is made up of plywood, louan, and 2x2s'.  So imagine a piece of plywood after a year of soaking in water.  The roofs on most of the newer models are plastic or fiber glass sheet.  They will detiorate in time due to the elements if not properly coated with sealant.  Best thing to look for is water spots on the ceiling and buckling in the walls, poke around and if your finger goes through the first layer of wood, then there is trouble.  The ones that are designed like the Airstream models(rounded edges and all metal) are probably the best built just from my observation. The ones that are made of plastic and fiberglass, I would stay away from unless you are planning on parking it under an enclosure.  Also before exchanging any cash, have the owner plug it up and test all the electrical appliances and plumbing.  Use common sense when it comes to the tires, age plays and important part on this, if the owner says it hasn't been used but a few times and it's over 5 years old, then the tires probably need replaced if it is to be moved any length of distance.  Look for cracks and unusual wear on either side of the tread, if wear like this is present, then  you probably have a bent axle.  Last but not least, make sure there is a clean and clear title in hand with no judgments or liens.  I hope this review helps, sorry if I missed anything.