1) Buying an item to resale- 73-77% of the PROFIT that you would make.
2) Buying an item to use- 83-87 % of the PROFIT that you would make.
3) Immediate Buyouts- Resale is to be at 80% and use is to be at 90%.
These mean if an item shows as 75g, there is a 7.5g fee and a fee of just over 3g. The profit amount is 64g. So calculate 64 * .75 or whatever percentage in that range that you would like. This to try to prevent the kind of people that would just abuse a guild member wanting an item. It prevents them from making a full 100% profit, but it still gives them a nice margin of gold to be made.