Re: Buying a Second Hand Horse trailer
I don't know what make it is, but I guess they're all pretty much the same.....
If it's an older one with a wooden floor, take up any matting and go all over it to make sure it's not rotting away with urine and weather - ones with wooden floors are also heavier to tow. With a metal floor, check it over as well anyway.
Let down and then put back and do up all the exits to make sure they're working ok and haven't rusted.
Make sure the tow bar works properly and hasn't rusted on - ideally attach it to your tow ball and take it round the block for a little try, then unattach so you know it does work and tows well.
Check electrics - brake and side lights and any interior lights.
Does it have one of those safety cable thingys - sorry, can't remember the proper name 
that puts the brakes on if the trailer should become unattached - don't worry if it hasn't, you can get them quite cheaply.
Tyres - should all have the right amount of tread, whatever that is, lol !! the spare as well.
If you can see the roof, has it rusted at all? If they're kept under cover it's a lot better for them.
Can't think of anything else at present, but I think that's the main ones - good luck :o)