1. Get independent comps from a licensed real estate appraiser...you'll pay a coupla hundred bucks...but if the comps aren't the same he's got some explaining to do...
2. If he also represents any buyer on the sale, you can negotiate 1-2% off his commission...any realtor would be a fool not to do so...and they all bullshit you about their agency allowing them to do this...but it's a common practice when requested...I even had a realtor or prospective new owner pay for pest inspections and sewer tests...
3. You never have to be pressured into selling. If you signed an agreement with him you can just ride out the duration. If his comps are bad you have grounds for breaking it.
4.Table the revenge until later. When your motive is less apparent.