I think this one is in The Bamboo Cradle – a girl needed to buy some sewing needles (machatim), so she went into a “kol bo” and asked for “mechutanim” (in-laws). The man behind the counter said “Do I look like a shadchan?! I don’t sell mechutanim!” She still didn’t get it, and said “but I saw some in your window. . .”
And of course the classic: “ani lo tachas omed”