If a client's house sells for $310k vs $300k, that's an extra ~$10k for the client. On the other hand, spending the additional time negotiating only nets the agent $300.[1]
Typically agents will sell additional houses rather than negotiate prices on sure sales. However their behavior changes when selling their own houses.
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[0]: From Freakonomics as mentioned by vitalique and roel_v below.
[1]: Real estate agent fees are typically 6%, equally split between buyer / seller agents.