Further to the above, I don't know any serious buyers who would be happy to buy a site without having a look into the site's GA (unless it's a tiny transaction or the value proposition is something other than traffic/revenue) - be it through guest access or a screen share.
You should of course do your due diligence when it comes to buyers and vet them a little, to make sure you're not showing all of your data to a competitor or someone who's only interested in replicating your business ..