Reproduction of a sticker calling for boycott of Jewish businesses (1934).
The text says: "Wer beim Juden kauft ist ein Volksverräter", meaning "Who buys from Jews is traitor to the people"
Eager supporters of NSDAP would stick them on the door of the individuals seen shopping in Jewish stores. 
 
Sticker is roughly 6cm (2.5 inch) in diameter