Well,I won't say this was mine idea.But I've seen it from other sites and it seems interesting.Here are the rules:
A member that doesn't want a character anymore can sell it instead of killing it.You just write the names here (it would be good if there was a link for the form) and if any member wants the character,he/she pms the member that sells the character so they can decide about the price.You can't use a character you are waiting to be sold.
Easy enough?I hope so!