The way I remember "Käufer" is the phrase "Cough it up" used when you're asking for money, because Käufer sounds like cougher. The person "coughing it up" is the "cougher", or Käufer. Maybe that can help someone else remember :P
Koffer is ransom in Hebrew. Easy for me :)
Thanks for the hint, got it remembered already.
"Einkäufer* should also be a viable option here.