You're probably going to need a real estate attorney to look over the purchase agreement and other documents and notices and determine who are the culprits and send them letters demanding the damages you choose - the benefit of your bargain or return of your down payment. If the house has gone down in value, the return of your down payment may be your better option. Unfortunately, it may be difficult to obtain this if the party who took the payments is gone or does this routinely.
You can try to recover all of these damages in small claims court, but you have to prove your case on all of these issues with evidence in a very limited time (often ten minutes or less in some crowded courts) and you can only get up to $7,500 in a small claims court action.
Letters should have been sent by certified mail, return receipt, giving thirty days notice. Without that, it is your word against your landlord's that you gave notice. The lease may also provide how notice must be given and where it must be sent, if by mail.