I've seen many examples of this sort of bulbous, boxy, yellow sign over the years. Illuminated from within, the most common varieties advertise groceries or pizzerias through the use of symbols such as a grocery cart or slice. (My vague guess is these sort of signs became popular in the 1940s and '50s.)
But this is the first I've seen that proclaimed the presence of a stationer. The silhouette of an inkwell and quill pen is quite specific and lovely, I think. And it took me a while to figure out that the top figure was a rubber stamp.
This is a ghost sign, by the way. Bay Stationers is no longer on Avenue U in Sheepshead Bay. The address is now a ballroom dancing joint. I'm told the company still exists, but doesn't have a storefront and sells directly to offices.