To a marginal miner ... someone paying average residential rate of $0.15 per kWh, for example, the efficiency of the ASIC hardware is crucial. An obsolte S9 is worth $0 to them, if their rationale for mining is to earn a profit, as they lose money with each hash.

Whereas to someone mining using "free electricity" (e.g., electricity is included in the rent), then an S9 is valuable to that person as it generates, today, bitcoin at the dollar value of about $90 / month. They may be willing to pay hundreds of dollars for one.

So less efficient hardware will fluctuate wildly when difficulty rises faster than the BTC/USD price. Those S9s selling for $500-ish today were selling for $25 two years ago, when BTC/USD was under $10K and miners were losing money on them, and dumping them for more efficient models.