The tan coloring as stated in the description can't be relied on as being the actual color of the turtle as silt/slime/algae deposits on the shell that you don't see when the turtle is wet does show up when the turtle dries off after basking.
I'm thinking the turtle is either a trachemys or pseudemys since the description on the site estimated the size as being well over what you would ever see a chrysemys being. And Trach. and Pseud. are more commonly kept pets that people have no fear of releasing into the nearest body of water once they get bored with them regardless if they are native or not.