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.
An assignment must be clear and unequivocal; There must be clear evidence of the intent to transfer rights; An assignment must describe the subject matter of the assignment; Notice to the obligor.
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.
Contract Law For example, if A contracts with B to teach B guitar for $50, A can assign this contract to C. That is, this assignment is both: (1) an assignment of A's rights under the contract to the $50; and (2) a delegation of A's duty to teach guitar to C.