I will explain this using finance and economics.
Let B be the price of any arbitrary equipment "E" transacted in the Buy N Sell section.
Let P be the open-market average price for brand new "E", at authorized or lemon-covered dealers.
Risk-protection premium "R" = P - B.
For any R exceeding the buyer's risk-adversity tradeoff, the buyer would elect to buy at B (BNS) because it is the risk-discount is more than the risk-protection-premium.
R depends on how stringently the warranty can be "enforced" or how rigorous the warranty regime is. if the warranty protection is very strong then R appears to be more "value"...reducing demand in BnS.
but 99% of the time BNS is without warranty, so you are not comparing apples to apples, or maybe i just dont get it.