First - noted above is a 2013 Z/28 failure - maybe a typo on year, but no 2013 Z/28's.
On topic - valve and lifter issues are what plagued the Z06 - there are few documented cased of the same on the Z/28. Was there a fix after 2011 or not is much debated and no one knows for sure. But it is clear if you plan to track the car more than occasionally that it would be wise to upgrade the valve train to eliminate the risk once and for all.
Do a quick search and you will find this has been well documented. Here is a link to a thread that addresses your concerns in detail. There are links in this thread to multiple articles on the topic.
Link:
https://www.camaro5.com/forums/showt...Z06+valvetrain
Otherwise the LS7 is known to be pretty much bullet proof! Very few documented cased of issues with LS7 in Z/28, linked to the issues noted in the thread and I have no concern at all with mine for the street or spirited driving. If track it more than a few times per year and it was not under warranty I would do the top end - which is my plan later this year when the warranty runs out.
Best of luck with your sale - your buyer should not have any real concerns - as you know the car is awesome!