I will offer a non-expert reply pending the experts weighing in. Your cylinder with proper fitting still would create no better than a mismatched piece rather than recreating an original gun. I would rather have a converted but original all matching Mk VI than a mismatched piece, restored to "original" in chambering only. Also, strictly as a shooter rather than a collector piece, with judicious loads, the 45 ACP would be acceptable and most probably cheaper to shoot. As to eBay, "one rose doesn't make a summer'. Further, buyer motivation is an important factor. Perhaps the buyer had no cylinder at all. That would represent a distinct added value factor for such a situation.
Though not speaking directly to your question, I would begin valuation by asking what a complete non-matching Mk Vi in original chambering would be worth compared with a non-matching converted model. Perhaps your cylinder would logically be worth the difference in value, if any. That would be my approach to such a valuation.
Serialed part replacement with non-matching numbers does not restore originality. For a collector piece, the situation becomes problematic.