Once an invention is made public (regardless of whether it is sold or not), you cannot apply for a patent. Of course, you can apply for it procedurally, and you may even be authorized. However, if someone else uses your public information, your patent can be invalidated through legal procedures. If it is an invention patent, during the substantive examination stage, the examiner may discover that your invention has been disclosed before the filing date, and then it cannot be authorized.