personally I dont think you can realistically expect altruism from all members and a certain number will have a primarily mercenary outlook. I include myself in as far as that because my purchases are based around a hobby (nothing more; its not like I pay the mortgage with gigging) and so I feel the need to sell for the maximum possible, and buy the best value possible because at the end of the day I have to have enough spare money to feed the children and keep a roof over their heads. If person A sells for too low an asking price and their item is swiftly resold by person B for more, possibly a lot more, then I don't have an issue with that. They should have asked for a higher price or sold elsewhere for a faster result. Regrettable but also avoidable.
I do think however that if person C sees person A selling for too low an asking price but doesnt want it themselves then maybe a quick PM nudge would be helpful to avoid sellers regret.