How are they destroying the market?

They assume the risk of the piece not being able to be sold, market price deflation, marketing, all CAPEX and OPEX required to run the business so obviously they will low ball the offer for a piece and resell it higher.

Nothing stops anyone from doing that, in fact many do do that. I've personally more than happy to let go of a few pieces that appreciated like mad for no reason whatsoever, and the market for the same piece now is back to near my original purchase price.