Although I agree with the overall premise of the OP, I disagree with some other comments in this thread.
For example.....
1.) Just because something hasn't sold after 25 bumps doesn't mean it's overpriced. It might just be an expensive item and nobody of the board can currently afford it, or currently has interest. For example, when I parted out my rack rig in 2011, I probably bumped my thread 40 times. However, I never needed to lower my prices to ultimately sell the gear. It was selling slowly because it was all high end (e.g., Bogner Fish, Soldano X88R, Egnater M4, etc.) and, therefore, expensive. Contrary to popular belief, a lot of people aren't comfortable or quick to shell out $2k on gear. Anyways, it all sold. "Bump count" was irrelevant to the prices being asked.
2.) When it comes to high end gear, the concept of 'market value' tends to go out the window somewhat. Sure, everybody has a general feel for what some of this stuff typically sells for, but if someone has, say, a MINT used 20th XTC and wants to ask 25% off the new price, what's the big deal? Someone out there might be ecstatic to get a next-to-new amp at 25% off the cost new. The bottomfeeders will wait for one to pop up at 50% off. The bottom line is, when you're dealing in expensive, high end/boutique gear, the 'market value' comes down to what a purchaser is willing to pay, and every purchaser has a different threshold of 'reasonability' when it comes to used prices.