A quick reply as im off out soon.
1) check for any rips down by the base of the hood, roughhly where your shoulder would be if you were sat in the car. This happens alot.
2) check the top right and left corners (inside and out) above where the Rear window is. On loads of roadsters i have seen the fabric is pulling away at these corner points. Inside you will often see the material pulling apart and patches applied. Mine waslike this but held strong for 2 years but on some you can see daylight.
3) check for any rips or issues with the seals, especially over the window. Quite a few members have had leaks because of the seals andits not fun and they are extremely expensive to replace.
4) check the footwells for evidence of leaks, often my leaks would come under the seal down the door then back in to the footwell. In all honesty if i was buying a 350z roadster again id get a hose and flood the roof and check for leaks. Best to do it ow as it totally ruins the experience afterwards.
5) repeatedly check the roof mechanism by opening and shutting the roof, it is alittle clunky but it should work with no issues. Sometimes when cold it will stick but thats a common problem.
My adviceif you see anything that doesnt look right or slightly damaged either walk away or bargain for a big discount. Roofs are not cheap or easy to fix/ replace, do your homework beforehand to avoid any issues.
Prime example check out some of twobears posts, she is not having much luck with her roof
I would also get a committment from them that if there any issues with the roof, either the material or the mechanics then i want them to fix them. I did it on both my zed roadsters andim glad i did as both had problems that the garage agreed to resolve.