The mandate must be signed by the seller(s). If there is more than one seller, then all must sign, or there needs to be consent or a Power of Attorney giving authority to the person signing the mandate.
The practitioner also needs to sign the mandate, as it is an agreement between the seller and the practitioner. If the Property Practitioner is an intern, then the mandate must be signed in front of a full-status practitioner or principal.
All sole mandates, sole and exclusive mandates, shared mandates, and multi-listing mandates need to be documented in writing and signed by all parties; these are binding on those parties. An open mandate can be verbal and is not binding on either party.
Suggestion: If you are taking an open mandate, it is advisable to get some form of written agreement regarding the commission that will be payable should you sell the property. The commission should always be agreed upon BEFORE you take on the responsibility of selling the property. You and the company are going to spend money BEFORE you have received anything, placing you at greater financial risk than the seller. You want to know what you are working for before you start.