Any markings on boxes should be obliterated by using a black marker or some other method. If they can be marked out, then you can use the box.
The clerk should have known proper procedures, and should have advised you at the time of mailing.
People reuse boxes all the time and that's okay as long as the outside of the box does not indicate that the contents are something they aren't. For example, any type of alcoholic beverages cannot be mailed, or some other product that might be considered hazmat.
Since the vast majority of parcels are sorted by machine, it is imperitive to know that any potential hazardous material is handled properly, so it doesn't cause safety issues.