Could be very reliable if it was well kept, you can get a feel for that if the owner is forthright about what they’ve fixed and maintained.
14 years outside, the roof could be trashed, or 14 years stored under cover and out a couple months in the summers is a different scenario.
As you know there’s kinda 2 separate components, in general. The house and the chassis.
In this case the house is presumably well used. (A dozen years as a “summer cabin”). Could be super easy use if it was a couple old people who spent a month a year there or could be worn the f out if it was a family with 4 kids and mooching cousins who spent all summer every summer in it.
The chassis should be like new save for a little maintenance but that’s of little value to you if you’re just parking it too.
Personally I’d be targeting one miles on it and light use on the house rather than the other way around.
But it’s impossible from your description to tell if this trailer is actually a good value.