Sell used diesel engines on Diesel Engine Trader or eBay. Diesel Engine Trader attracts over 5,000 visitors a day and allows buyers to contact sellers directly. While the website is available internationally, most visitors are from the United States.
Diesel Engine Trader listing fees start from $10 per month, as of 2015. The website allows sellers to include pictures in their listings for free.
Alternatively, sell used diesel engines on eBay. EBay offers a dedicated subcategory for diesel engines and further categorization based on the manufacturer. You can sell your diesel engine at a fixed price or put it up for auction.