Long before NASA's Curiosity rover ever existed, the US sent a couple of intrepid missions to Mars thanks to the Viking program. Viking 1 and Viking 2 reached Mars orbit in 1976, way back when bell-bottoms were in style. Those orbiters were responsible for delivering a pair of landers to the surface. A set of engines with the designation RS-2101 helped the spacecraft reach their destination and settle into orbit. You now have the rare opportunity to buy an unfired flight-ready spare engine designed for the Viking project.
RR Auction describes the engine as "as close to a flown engine from the project as you will find." Essentially, the company is daring you to pop over to Mars to try to pick up a flown engine if the spare isn't authentic enough for you. Good luck on that.
The engine was made by Rocketdyne, a division of North American Rockwell at the time, and consists of materials like beryllium, niobium and stainless steel. It is 22 inches tall and sports a 10.5-inch nozzle diameter. It seems sized just right to turn into the world's spaciest coffee table, though no one would blame you if you decided to use it as a bedside table to inspire your dreams of space exploration.
The RS-2101 is currently in a phase called pre-live bidding, where early birds start to run up the price. It started with minimum bid of $5,000 (roughly £3,070, AU$5,492) is now up to $11,713 (about £7,082, AU$12,854). Live bidding starts on September 18.
Purchasing this rocket engine may get you just slightly closer to your ultimate goal of building, launching and landing your very own private Mars rover. You'll probably need a couple more billion dollars to finish the project properly, but at least you'll have one of the engines for your orbiter sorted out.