Employers and employees have by their nature a very trusting relationship. The employee trusts that the company will keep paying them, and the employer trusts that the employee will do the job that they are being asked to do. The employee also trusts that the employer will act professionally. This trust includes NOT selling the employees personal data to the highest bidder (or any bidder for that matter). Companies out there do this to their employees, and they shouldn’t be doing it. The small amount of revenue is not going to offset the loss of trust that employees have in the company they work for.
Selling customer data is a horrible enough practice, but companies can stand behind their EULA which probably says something about the person using the service is the product to be sold, and the actual customer is the company that is buying data (you didn’t think Facebook wasn’t selling every bit of data they gather?). Selling employee data is a whole-nother mess to deal with as employees are going to have no way to opt out of it, and no way to stop it. Even leaving the company isn’t going to help, because once the data has been sold, it’s gone, and you have no idea what the purchasing company will be doing with it (here’s a hint, they’re probably going to sell it).
If you’re someone who’s in a position to approve or deny these sorts of sales of employee data, the answer is “no” — every time, with no exception. Your employee data shouldn’t be for sale, and I’m guessing that your employee base is going to be some pretty low-value data to purchasers, as once people find out how/why the purchasing company got their data, I’m guessing the people won’t be purchasing anything any time soon.