Copyright on old drawings?

I am not sure if I am posting in the right place but I am wondering about copyrights on old postcards and greetingcards. I don't understand how people can charge for these images when they did not draw or paint them and claim they are copyrighted. These people usually get them out of a shoebox in thier grandmas attic. What if I had the same greeting card? I am pretty sure more than one were produced. Can someone explain how this works?