Must you use an online service?
You might do equally well -- perhaps better with a traditional auction house.
If the artist is sufficiently well known, even one of the big names like Sotheby's might be interested. They have clients with deep pockets.
Many auction houses now have live online bidding, which would fulfill the donor's requirement.
You will pay fees to an auction house. But you also pay fees to eBay, to Paypal, and have that pesky 21 day Hold.