The assignor must agree to assign their rights and duties under the contract to the assignee. The assignee must agree to accept, or "assume," those contractual rights and duties. The other party to the initial contract must consent to the transfer of rights and obligations to the assignee.
If you communicate clearly, then this is typically not an issue. This is because the seller has signed a legal and binding contract, which makes them obligated to fulfill the terms of the contract. You have every right to perform an assignment of contract when everything has been done legally.
An assignment of contract is a legal clause that allows for one party of a contract to transfer the rights, obligations, and responsibilities of that contract to another party. The party who is giving away the responsibility of the contract is the assignor and the party receiving is the assignee.