All the Jayco hybrids are essentially the same beast, just different sizes and floor plans. We in our our 4th summer with our 2013 X20E. Our previous hybrid was an '03 Jayco Kiwi 17a. We had that trailer for 10 years, and got a decent trade allowance for it because of the condition it was in. Jayco makes a solid product and the usual bugs that trailer may have had new are most likely worked out. My beds on either hybrid have never leaked, though our old trailer did develop a small leak on a roof seam where the Dicor dried up and cracked. It was an easy fix and no water damage was done. This is a potential issue on any trailer not just a hybrid.
Look for evidence of leaks. Not just around the beds, but the roof, ceiling, walls, around windows, etc. If they allow you to, get on the roof and inspect all the seals. Have them open the beds and inspect the bed seal. It should be soft and pliable. If you are serious about this ask them to demo everything for you so you know it works. Ask them what they would fix if they kept it. Most people will be pretty honest about that, especially if it looks well cared for.
$8500 seems a bit high for a 9 year old trailer that's on the smaller side. Make sure to check the
NADA guide. It might be worth every penny they are asking, it might not.
EDIT: One other thing to add, check the date code on the tires. ST trailer tires should be replaced every 5-6 years regardless of how good they look. If they are in need of replacement or close to it, that might be a good thing to use in haggling.