Yeah - as people mentioned, remove your Adsense code.
I flipped a site a few months back, and literally a week after the sale Google killed my Adsense account (I only used the Adsense account for that domain, nothing else, as I use AdBrite on everything else). The reason for Google killing off my account is because I was a "danger" to their advertisers or some nonsense. Appealed it, and they upheld the ban.
When you flipped the site did you keep the adsense code on it during the sale? and how did you sell it please? Flippa, DP, etc...
I'm considering selling some sites with adsense and I'm interested to know more what happened to you, I would just prefer to keep the ads running as an example to show the domains are adsense approved, but certainly don't want to risk my account.