Ok here are the guild bank rules!
when you donate an item to the guild bank (starting 24hours after the Event: Bank Auction) you get 50% of the points donated to the guild bank!
here is how you work out the points value of an item:
Item Level(Requires Addon: Auctioneer To See) X Item Rarity(Grey=1, White=2, Green=3 etc.) Divided by 10= points value (rounded down to the nearest whole number with a minimum value of 1)