I once got a frame that had a perfectly good voltage regulator/rectifier still attached (from Roy at "Bikes In Fields"). I didn't mind the bonus, and he didn't mind me getting the part. Since I swapped him a perfectly good and running bike for the frame (and brakes, and wheels, and forks for an XS 650), we were both happy. A win-win.
The only surprises I have heard of is when the frame vendor decides to withhold the swing arm and shocks. I think that is is bit much. Those are essential components of a "frame" in my opinion.
Communication is always the best motto. What was your purchaser expecting to buy, and what were you expecting to sell? No hard feelings where there is a meeting of the minds.
Peace & Grease, Dennis