A seller posts a chip on this board for sale. He asks $100 even though the regular ebay price is putting the market at $25. A buyer will be paying a 300% premium for being unaware of this.

If the seller does not know about the ebay and other market sales, should we tell him to give him the chance to adjust? Do we have an obligation?

If the seller does know about the ebay and other market sales and is trolling for an above-market price, is that wrong? Must sellers offer at/near market to be 'fair'?

IMO that is why we have a marketplace. Information and competition are better devices for fairness than some ambiguous 'higher morality'. If you are selling and don't want to lose out, do research. If buying, same thing. Good sellers will prosper -- bad ones will be avoided....