Hello, Dusty:
Been down that road.
The question may be more about how to assure a buyer that your barrel performs.
I have had barrels, not repro-Spencer's, that had issues, but shot well; even won medals. Proof's in the pudding, so to say.
If a prospective buyer is concerned, one way to eliminate doubt would be to supply a test target you shot with all info. Then give the buyer mutually agreed return privledges. Yes, who can say if a buyer will duplicate your results. Otherwise, your barrel and chamber would have to be inspected by a qualified gunsmith for internal dimensions. Even then, that's no guarantee of accuracy.
All the best,
El Supremo/Kevin Tinny