Simple Seller is Free Software and Open Source Software. Or, as they say, Free as in Beer AND as in Speech. This means that it is free of charge for you to use, and you are free to share it, modify it, and use it as you see fit (but of course, you must comply with the GNU General Public License which covers it).
However, if you find it particularly useful and want to give me a little token of your esteem, who am I to deny you?