If you have evidence or proof that they have the shed--which can be the testimony of a witness who personally saw the shed moved to or at their land, BUT that witness would have to appear in court and testify, since you can't, under the rules of evidence, testify as to what someone else told you outside of court; or a photo you took of the shed on his property--you may be able to sue for "theft by deception," or stealing by tricking you. For a $3,000-ish shed, a good option is to sue in small claims court, acting as your own attorney, or "pro se," to save on legal fees. They key is, you need proof or evidence, not rumor or out-of-court hearsay.