Donation for tax deduction has many requirements attached and is not the answer for moving old iron for better than scrap metal prices. The value of any machinery is based on its condition. The better way to say that is how long the machine can still produce revenue or how much money has to be spent so the machine can produce revenue.
Iron Planet will require an inspection that will reveal most if not all the problems areas of the machine. Ritchie Bros. doesn't care about any problems. In auctions which have no reserves you have absolutely no control over the price and you still have to pay commission and incidental costs. You can set a reserve but you will likely not sell the machine. If you need money quickly, auctions are the way to go. If you need a certain amount of money, you are likely to be disappointed with an auction.
What I might suggest is finding a different dealer with a decent reputation and see if they will consign the machine. It will take some time to sell and might require some money in making it operational but that is probably the best you can do.
What everyone seems to forget is that equipment makes money and is used up by operation. After you have used it up, you have maximized the money you can make on it. Now you have to liquidate it at your expense. That is why so much of it just sits.