Just looking on the flip side... If something sells in 2015 for $600 and is now worth $1000 then it cannot be dug up and used annoyingly in 2021. Also I feel it is respectful to keep the sold price between parties. A third thought would be most collectable aged items are unique to themselves for condition. Each item has its price and the price is what someone will pay. If I sell two MM's Black face but one has the slightest imperfection and the other is perfect I do not want to deal with someone claiming I am overcharging cause I sold the other cheaper. Not against the idea of leaving the sold price after the sale but am looking at the other angle of it.