via　https://www.cea.gov.sg/docs/default-source/module/newsletter/1-2017/dos-and-dont

The don’ts for co-broking

#1: No blocking of other agents

If you are representing the seller, you should not deny co-broking opportunities to other agents.

For example, if you receive a call from other agents, you should not tell the other party that you are not sharing your commission, which is indirectly blocking the other party from co-broking.

As co-broking exposes the property to a wider pool of interested buyers, the greater exposure could attract a higher offer and eventually a higher price. By rejecting to co-broke with other agents, you are not acting in the interest of your client.