I like Meredith's comments, it's an organized plan. And, an organized plan will make sure you get the trailer you want.
As for the trailer in the ad, one can't see much from the photos he posted. I would ask for much larger photos including the kitchen, and ask what appliances have been changed (refer, furnace, water heater, AC) and when. No matter how nicely it's been kept, all those things will very likely all need to be changed if not immediately, in the near future. They just will not live forever. Neither do tires and running gear (brakes, bearings, axles etc.)
For the price he is asking, I would expect them all
to be fairly new. The exception might be the AC unit; they seem to be able to last nearly forever, and if it's the original Armstrong, can almost always be repaired or rebuilt.
The interior he shows is original. Same comment for the fabric as made above re the appliances: it does not last forever.
All in all, I would say that for what he is asking, I would expect tip top shape, and I don't see that in the limited phtos. There's a kiss on the front streetside that doesn't look pretty, for example.
By the way, welcome to Airforums.com! What you are doing is just one example of how we can help you along the way with your trailer. There are also people willing to inspect them at reasonable fees, or free (I would at least offer fuel costs if they are not fairly close to the triler being inspected.) You will see an access panel to find one near you on the portal page, the one you land on when you login.
Also, as Meredith mentioned, there is a Classified section on here. Be sure to log in using your current user name/password that you use here. You will see a link to it on one of the top horizontal bars with a white background.
Good luck, and keep asking questions!