When we bought the intellectual property of a competitor that had gone bust, it came with the customer database. However -
We kept it as with the nature of our work data has to be retained for a minimum of 40 years, and in some cases permanently - so we are continuing the extinct company's duties as it were. We have had a few of their clients contact us with regard the work that was done for them, in which case we then request a new agreement, but we are very careful not to use any of it for marketing purposes.