Artemis:
donations can have the amount kept secret between you and our 'banker', but an auction doesn't work that way. I've participated in many silent auctions and the bid amount - including the amount of the winning bid - has always been visible. How does anyone know by how much to outbid, if the amount isn't shown?
Also, depending on what people who are offering 'swag' decide, there may well be a minimum donation required to qualify for the item they're offering.
Wendy
