Ok. I have a very messy situation. I am a manager of a department that deals with company money. I recently learned that one of my team members bounced a check for a fundraiser she brought in from which other employees ordered including me. The employee told everyone a series of lies regarding the order status with the latest being the school is supposed to refund the order because she canceled it. I know the school has the merchandise and is awaiting payment from the employee who bounced the check. I notified HR and my supervisor, but both said I couldn't do anything about this because it is not work related. I disagree. Isn't this classified as fraudulent activity? My company is in an at-will employment state. By not acting at all, aren't we sending the message to the other employees that "we tolerate theft as long as it isn't company money"?
What do I do with this messy situation?

NO, you may check company policy about selling things at work, but this should have been done on personal time like breaks, ( or all employees would be guilty of personal activity on company time)
But this is one employee cheating others out of money, You could not as a boss but as a person they owe money to, discuss this that you know they are lying and that they need to correct this. But no as a boss you can not say anything. This is a issue of mixing personal activities at work.