Every case depends on the details but IMO there are certainly some things you can and can’t do. Others may have different views.
I think the short answer to your question would be no if someone has something they make and I’ll assume are selling you wouldn’t be able to patent their creation. For you their creation would be seen in the eyes of the US patent office as being prior art and prevent you from getting a patent since this creation was already being made by someone else before you filed your patent. Even though you would be the first to file the creation would be classified as in the public domain. The person who created the item would have a short window in which they could make and sell their creation and still get a patent.
Can you change something enough to make it novel and non obvious and get a patent? The answer would be yes but that can be a hard sell when you try convincing the patent office/examiner your new creation is novel and non-obvious. Every case is dependent on the details and to make an informed decision you would ultimately need to contact a patent agent or attorney.