There must be a whole lot of special event chairs who would love to have your problem. I am not sure why you think it would be unethical to auction a few tickets if there is actually a limit on the number of people who can attend the event.  Everyone knows that the whole purpose of a fundraiser is to raise money for the organization and they are all willing to pay more than the event is worth in order to support the organization.  Many fundraisers have tiered pricing so that people can contribute at different levels even though they all get the same chicken dinner. I don’t know why anyone could legitimately object to auctioning a few tickets to raise extra funds for the organization.

If the event is as successful as you say it is, however, I have to wonder why you don’t raise the price of all the tickets.  It sounds as though you aren’t maximizing your potential revenue.