As an agent you make your money on the overall volume, not squeezing the last bit out of any particular deal. Near as I can tell getting a reputation for deception is nearly career suicide unless you're at the very low end of the market. I think you're making a fundamental mistake in thinking about incentives in the context of a single sale vs having a career in a business that is very strongly driven by personal networking. This is a common fallacy in microeconomic thinking.
I don't doubt that two agents who had frequently worked with each other amicably might be more frank, but I do doubt that agents all, or even mostly, presumptively do this with every counterparty they meet.