If you have any document showing the receipt of any money by the owners by themselves or through the agent then you have evidence of an agreement enforceable in law and you may file a suit for specific performance of the agreement.
In case you are comfortable with letting the sale go and recovering your money then you may file a suit for recovery of damages alleging non completion of agreement on behalf of the landlord causing you irreparable loss and damages.
If you have no document or receipt from the landlord then you may have to move for recovery of damages on the property dealer because the court will not be able to enforce specific performance on the agent where the title to the property belongs to third party. You pray for damages as well against the agent. .
If however the agent runs his business in form of a company through that company then you night also consider filing a winding up petition because a winding up petition is normally disposed off within months whereas suits take many years to be heard. But please do note that in a winding up proceeding you may recover the original amount and some interest but cannot claim damages. Winding up petition will include some technicalities and hence you will be better advised to check all four corners of law before proceeding on the form of remedy you choose for yourself.