Always a tricky thing. The bike charity co-op in NYC that I dropped parts off at before moving back to KC, they would go through more desirable parts and sell online or in their retail location with funds going to the operation. So maybe just looking for the right co-op that does that sort of thing is an answer, or put that stipulation on your donation?
I've also seen people who will sell items at market value or maybe below and denote that proceeds are going to charity, kinda like a bake sale. I'm personally fond of that approach and will be doing with some odd and ends later in the year.
You could also offer up on a forum and state that the buyer can donate the dollar amount of their item to a list of charities and provide proof, then you can provide them the item. I use a software product that the owner calls charityware, where you can only obtain a license for the product by providing a receipt to a charity of your choice. I've also seen raffles done recently like this.
--ben in KC.