Technically, any signature that is not the cardholders signature is forgery and unenforcable as a negotiation....
However, if the cardholder (account holder) does not take a reasonable effort in protecting their card/account (ie, leaving black spaces in checks, giving their credit card out) -- they could be held liable for it...
Now, in the case with authorization... the person, would in effect be an agent of card holder ... with the powers of such. A note stating that they had permission to use the card, would suffice to show proper agency existed... it would be an appearence of agency, and that would be enough (even though an ACTUAL agency would not exist)...
I think, given the situation... if one person gave another permision to purchase 1000 worth of itmes, and the entrusted person purchased 1500 worth of items... the original card holder would be liable for the entire 1500 on the credit card charges, but would have a remedy against hte entrusted card holder for the remaining 500, which was above and beyond the agreement.
C-ditty