If property is properly abandoned, you can sell it to off-set the rent owed. If the owner wants his property back, holding it as collateral against the debt is going to be problematic for a number of reasons. You have an obligation to properly store and hold the tenant's property pending pickup in a safe location.
The foregoing is not legal advice, and nothing in the foregoing shall be deemed to create an attorney client relationship. If you feel you need to speak with an attorney regarding your issue, it is recommended that you contact an attorney with expertise in your area of inquiry. The information related above is purely for informational purposes, and should not be acted upon without speaking with qualified counsel familiar with you specific situation and the laws related thereto.
Pursuant to N.J.S.A,. 2A:18-72, et seq.,a landlord may only dispose of property left behind in the rental premises if said property has been abandoned by the tenant. In order to show abandonment the landlord must to give notice to the tenant, by certified mail, return receipt requested addressed to the last known address of the tenant (which can be the address of the rental premises) , notifying the tenant that the property is considered abandoned and will be sold or otherwise disposed of unless the tenant makes an appointment with the landlord to pick up said property within 33 days of the mailing of said letter. It is important to note that if the tenant does respond to said notice within the 33-day time frame, the landlord MUST make the property available for removal by the tenant WITHOUT payment by the tenant of any unpaid rent. If a landlord fails to comply with these requirements and sells or otherwise disposes of a tenant's property, the landlord can be held liable for double the damages sustained by the tenant. Any property located in the premises rented by the tenant should be considered to be subject to this law and therefore you may NOT sell or otherwise dispose of any property left behind unless and until you have complied with the law.
The best course of action is to have all items in the unit properly secured; provide the written notice as required by the statute (cited in another answer); and retain an attorney. The advice given on this forum is general in nature and only an attorney retained to represent you can understand the specific facts of your case.
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Did you have a court order to lock the tenant out? If Not you will be ordered by the court to let the tenant back in. You may not hold the stuff hostage. You have a duty as pointed out by other counsel to store the stuff for 30 days.