Can personal property be held as collateral if money is owed?

Since you don't yet have a judgment, you can't keep their property (you might be able to keep abandoned property, but from your email it doesn't appear that the property was abandoned in the legal sense) unless you have a court order or contract allowing you to do so (e.g. a mortgage) or the situation falls within a narrow range of statutory liens which are permitted when someone has performed services to improve or repair the property in question (e.g. a mechanic's lien) which doesn't seem to fit your situation.  While you claim that the other party owes you money, the other side may claim otherwise, and a court will have to determine the issue before you can lawfully take action against the debtor's assets to collect the debt. However, now that you have an address, you should probably take the opportunity to file a lawsuit and serve them with a summons and complaint.  If you wait until after you return the box, they may disappear again.