If you set an auction to “Guild Members Only” and then leave your guild, then the auction will not be able to receive any more bids (unless it was also set to "Neofriends Only") until you join another guild. If you join another guild, then the members of the new guild will be able to bid on your auctioned item.
If no members of your new guild bid on the item, but members of your old guild did, then the item will go to the highest bidder from your old guild.