You file a lawsuit against them for breach of contract (violating the agreement about how they will handle past-due payments and selling unit contents) and conversion (a form of theft; converting belongings entrusted to them to their own benefit). You would sue for the economic value of your belongings (you can't sue for emotional or sentimental value) as it was on the day it was sold. If the value is at or under the limit for small claims court, you are strongly encouraged to sue in small claims: it much simpler procedurally for a nonlawyer, and faster, too. You can get instructions and sample forms from the court, in person or online.