There are any number of record dealers who post clips on YouTube about large collections they've acquired. I would suggest you contact one of these dealers to ask their opinion of whether you have the right to sell the goods. If their response is encouraging, you then contact an attorney for a consultation about the legality of you selling the property. This could be a single consultation for which you would pay a flat fee; you wouldn't have to involve the attorney any further. I doubt a dealer would buy it from you if he wasn't sure you had the right to sell it.
Another option open is to hire a private investigator to find the whereabouts and contact information for the person who left the records with you. You could then negotiate with the owner, and perhaps get a written agreement to accept the the abandoned property in lieu of payment of the loan. This could also be done through an attorney, who would have established relationship with investigators. Whatever course you take, arrange a flat fee for the attorney's services, and then decide if it's worth it to you.
Another option open is to hire a private investigator to find the whereabouts and contact information for the person who left the records with you. You could then negotiate with the owner, and perhaps get a written agreement to accept the the abandoned property in lieu of payment of the loan. This could also be done through an attorney, who would have established relationship with investigators. Whatever course you take, arrange a flat fee for the attorney's services, and then decide if it's worth it to you.