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 consent to the person signing the mandate.
The agent also needs to sign the mandate as it is an agreement between the seller and the agent. If the estate agent is an intern, then the mandate must be signed in front of a full status agent or principal.
All sole mandates, sole and exclusive mandates, shared mandates, and multi-listing mandates need to be reduced to writing and signed by all parties and is 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 that you get some sort of written agreement to what the commission will be should you sell the property. The commission should always be agreed to 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.