I also suggest full blood chemistries, a CBC and cultures as well as a thorough visual exam. The seller may refuse to pay for the tests in which case the wise buyer would either pay for the tests or decide against buying the birds.
There should be a written contract clearly outlining the conditions of the sale. The contract can include a provision that allows the buyer to return the birds for a full refund if the birds prove to be of the same sex. Should the purchaser waive pre-sale the medical examination then the contract should specify a period of time to have the birds examined. If the birds are not healthy, the seller should pay for medical expenses.
It will be important that the birds realize that they are not humans or some other species of birds. Many pet Hyacinths can become good breeding birds provided they understand that they are Hyacinth Macaws