The company had exchanged contracts in 2007 for the sale of a site in Hove to Banner Homes, and another site in East Grinstead to Alburn (Minos) Ltd, both subject to the purchasers' plans for housing developments on each site being approved.
However planning applications were refused, and both sales have now fallen through.
Caffyns said it will now seek alternative buyers for both sites.
The dealer group has been battling to rebuild its business after enduring tough trading conditions in recent years.
It is also current the subject of a campaign by activist investors, who have called for changes in its management board and want the business to consider sale or merger in order to improve returns for shareholders.