I don’t think many on the CDS work like that. Have sold through VIVA a few times in the past and I was clear what price I wanted and I paid the commission out of the sale. The commission should be nothing to do with the buyer. The practice of putting a bundle on the asking price was started by the spanish many years ago. I think the only agents who do this are the kitchen sink one man band agents.
How it worked is the “correadores” would ask an owner how much they wanted for the house. Spanish logic being that if they are satisfied with that sum then anything above that sum they could wrangle out of the buyer was theirs. Then the British caught on and adopted it especially inland when silly Brits were paying 80,000 for a ruin worth 40,000.