Your sentence is understandable and acceptable but not as natural as it could be. I would say (as Jazz also did) that using "being" in your sentence would make it better. --> "I went from being a top seller to the top-performing salesperson" I would also say that replacing the word "terrible" with "under-performing" would make it more professional too. --> "I went from being an under-performing seller to a the top-performing salesperson"

Your sentence would be perfectly understood. You could also say, "I went from being a terrible seller to the top-performing salesman (or salesperson)."

You are correct. The sentence sounds natural.