No blowby on this engine is a big deal - worth a premium in my mind. On the ebay ad, you ought to demonstrate it with a video showing no tea kettling. Plus folks like hearing the engine noise.
With the transmission, you could note if there is any burning smell with the fluid and even include a picture.
Those two items - engine and tranny - would be the biggest sources of concern of a Cummins buyer. Your 10.5k price seems very reasonable, and a great deal if you demonstrate the engine/tranny are solid.