A good roof inspection is a must. I understand about 10 years is the life of a roof. It can be extended by proper maintenance during its lifetime. As others have advised, do a thorough inspection. Power everything up to confirm it works as it should. If not, then expect repairs or replacement and the purchase price should reflect such. Check the age and conditions of the tires and spare. About 5 years is a normal safe lifetime. Rarely do they wear out, they just dry rot. Expect battery replacement unless it is less than 2 years old. The seller should be aware of any issues and hopefully will disclose such if asked. There is no such thing as an "everything is perfect" trailer. Make a list of everything you find that isn't correct. It is a negotiation point.