The alienation of the exclusive right in the work

[Ирина Ивановна]

Dear forum members , help to understand the situation :

in negotiations to acquire the picture buyer together with the contract of sale and transfer of copyright need to sign a separate contract for the alienation of the exclusive right to the work. What is the difference between copyright and the exclusive rights ? And the more fraught with the signing of such agreements? Please help me with the point of view of the buyer , why he , the purchaser , can not be content with just the contract purchase - sale.
In the past century, and in the past decade , generally did not know about such legal case , generally sold on a verbal agreement , and now have to start to penetrate .

Thank you in advance .