I picked up this signoff from a local comedian, "Otto," who said it on a radio show -- Basically, he said, "Good bye - Guten Einkauf!"
Because in English, the word "bye" sounds like the word, "buy" -- its homonym. Buy, of course, is synonymous with "shopping ." And "shopping " in German, is something like, "Einkauf." I always found it so funny, I never forgot it and continue to use it myself!
-- Thomas Z., Germany
Given Americans' rampant consumerism, I'll bet that most foreigners or non-native English speakers think Americans always sign off saying, "Good Buy!" Pretty soon, they'll be saying, "Have a nice shopping spree!"