Hah. Hopefully "used" just means the original packaging has been opened etc., and not that the strings were actually mounted on a bass and played. But who knows...there are some freaky bass players out there...
Bass players buy and sell used strings sometimes, especially expensive one like TIs. If I buy an $80 pack of strings and decide I don’t like it, there might be somebody else out there who could use them. And unlike guitars it’s easy to swap and reuse bass strings.