Trailer miles are not the same as a motor vehicle miles. A moter vehicle, i.e. your tow vehicle, has much more moving parts than a trailer. Pistons, rods, bearings, gears etc. You trailer has only wheels, bearings, brakes and associated parts to worry about. In other words, IMHO, miles do not materially effect the value of a trailer per se. As a previous posters said, it is CONDITION, CONDITION, CONDITION that is important! A trailer that has never been used and set for two years, might be in worse shape than one that is five years old and has 100,000 miles on it.
I have a 1999 that has been towed for 70 or 80,000 miles which is in like new condition.
Also, as a trailer has no odometer, there is no way to tell how many miles it has been towed. If it is in good shape, the owner would be inclined to be honest about towed miles, while if it is in poor condition, the owner would be inclined to under-state the actual miles. In short, just evaluate trailer condition and don't worry about towed miles. You have no way of determining what they are anyway.