The realtor is not liable for your costs due to delayed possession unless the realtoe is also the landlord. The landlord may well be liable, however, for breach of contract (a lease is a contract), for not giving you possession of the space when he or she had agreed to--it is therefore the landlord, who has violated his or her obligations to you, to whom you should look for reimbursement or compensation, and who you could potentially sue if not paid voluntarily. It may be that the landlord in turn could look to recover compensation from the existing tenant, but that is not your issue or concern.