There is much to be considered when buying a used horse float. You should make a list of desirable features because the design of floats varies enormously. If your knowledge of equestrian matters is limited, talk to other horse owners.
Unless you know the person from whom you are buying your used horse float, it will be necessary to establish ownership. In the case of a float that is only a few years old, it may just be a matter of tracing its history through registration papers. But the bottom line here is to make sure there is no encumbrance.
It would be easier to buy from the horse float from a dealer. That way title is guaranteed and you will have some kind of comeback in the event of trouble. You should test the float by towing it before you buy. It is not unknown for hitches to fail under load and the consequences of such failure could be catastrophic.
Make sure all the lights work. If you take a cable converter you can conduct this test even if the plugs on the horse float don’t match those of your vehicle.
Check for rust, especially in the floor and the underbody. Avoid a badly rusted horse float, regardless of how cheap it is. There should be a separate door on the side for access without opening the tailgate. How easy would it be for you to load and unload your equine friend(s) into the horse float without help?
You will need to what your horse weighs. Or you may be buying a float that can take more than one animal. With two horses, you are going to have to consider your tow vehicle carefully. I can mention the example of an older Land Cruiser diesel that struggles to cruise at 80 km/h when towing a float with two horses and gear.
With such heavy loads involved a float should be fitted with heavy duty wheels and tyres, not the items left over from an old Ford Cortina.
If you are buying a two-horse float, make sure the divider does not go all the way to the floor because this cramps the horses’ freedom.