Yeah, my suggestion is get it valued, check with a lawyer just in case (though you should be in the clear), sell it, and then pay your parents back for the rent they paid for him and keep what's left. If they think he should get the money, they can give it to him. If he thinks he should get them money rather than it covering his rent, he can go to your parents for it first.

(I do hesitate to say they should keep "market rent" because, well. Landlords. Market rent. But they should at least keep enough to cover reasonable costs for the time he was in the house, incl. maintenance, taxes, utilities, mortgage, and their time.)

That said this is so close to the plot of *several* different mystery books I read in elementary school that I deeply wonder if it's real. (I also find it hard to imagine where a comic book could be hidden in a secretary desk for years that would leave it in VF+ condition, and you aren't getting five figures otherwise.)