A new transaction service has popped up in the past couple months that looks promising for use here. It's called Gunpal (www.gunpal.net) and really doesn't care what you use it for:

"GUNPAL does not discriminate based on the nature of your transaction, requiring only that the merchandise or services you buy and sell be legal."

I think at most you'll have to send them an e-mail letting them know that you'll be accepting donations for site upkeep, but aside from that they could care less.