I really don't think it's neccessary to change the format. If someone doesn't want his e-mail address shown in public, he simply doesn't have to include it with his post.
The only time I include mine is when I'm looking to trade or other neccessity.
Maybe posters could replace their e-mail address with their known bidding nickname.
When needed an e-mail address could be included in a posted message. Spammers probably don't take time to read them.