Wow, this went sideways in a hurry...lol...
While I agree that what the current owner owes has nothing to do with the value of the trailer, I think the majority of people who owe money on a rig they are selling would try to recoup as much as possible to clear their loan... If that's not what you want to pay then walk away, the price will come down as the current owner sits on it and realizes he is not getting his price...
That being said, if the trailer is in immaculate condition with no damage and all maintenance is up to par then it is your call, you have to be happy with it in the end, no one else does. I would rather pay too much for a well maintained used trailer that has zero issues than take a chance on a "good deal" with questionable maintenance. There is nothing worse than hitting the road with your new to you rig and finding out you just bought someone's nightmare....