A Banker's draft is a cheque issued by a bank itself.
People often use a banker's draft when making large purchases, like a car. They get this by transferring money out of their account to their bank. In return the bank gives one of their cheques (banker's draft). This means that the draft will not bounce.
But if people are selling something, they should never hand their "goods" over until their bank has confirmed the draft has gone through, and that they have cleared funds in their account. This is because the banker's draft could be forged or stolen. Banks generally charge a fee for issuing drafts.
Please see 'Understanding a bank draft' for more information.