Translation:I am selling metal and plastic things.
it is the person is selling things that are made of metal and things that are made of plastic (it is more like "I sell metal things and plastic things")
I have no idea how to translate only selling things that are made both of metal and plastic, You could say "Sprzedają rzeczy zrobione z metalu i plastiku", which is literal translation of I am selling things made of metal and plastic, with the same ambiguity.
I'm a native speaker of English (AU). The two words have slightly different nuances. "Stuff" can show less respect to the objects (your stuff is all over the floor), or be used to refer to something that isn't countable (you've got stuff all over your face), or is a less formal term (I do stuff on weekends).
Really, though, the two words are very similar. You should use "things" if possible, though, because "stuff" is one of the things that we're taught not to say in school.
I will give these words in masculine form (commonly used in dictionaries)
plastikowy, rarely plastykowy - made out of plastic (material)
plastyczny - quality of something that can be molded, is malleable, flexible; or when one speaks about arts: something that has connection to works of painters, sculptors, illustrators (but not with music, literature etc)