Firstly, the answer to your big question is ‘no’ someone cannot come behind you and get a ‘utility patent’ on a device that you have been selling! Because you have been selling it your device is automatically in the public domain one year from the date you first disclosed it. ‘IF you don’t file for a patent during that year.
On the other side of that coin is that without a patent you have NO legal recourse to stop ANYONE ELSE from making and selling your exact product.