Nothing, really. When the "a" gets an umlaut, "ä," you either pronounce it as "æ," like in the American English pronunciation of "apple" and "e" like in "bed." It's personal taste, though the latter is more common in Germany. Since "ä" is close to or IS "e," it becomes an "ɔi" when paired with a "u."
consumer is a correlate of Konsument and is most often rendered as Verbraucher in German. The term designates a person who is the last to buy and use (use up = consume = verbrauchen / aufbrauchen / verzehren)) a product, hence its meaning (at least in German) is more extensive than Käufer. YMMV.
Nice idea, but coffer is from French 'coffre' meaning chest or box, and is actually very literal from the days when people would keep their money in chests. And cough is from German, but from 'keuchen' meaning to pant, rather than 'kaufen' . 'Coughing up' meaning paying reluctantly is from the idea of it being unpleasant to literally cough anything up.
Could "the shopper" also be a translation for this? Is there another German word that would better fit the meaning of "the shopper"? If it does make sense to translate der Kaeufer as "the shopper" then I would suggest it be shown as one of the translations when you hover over Kaeufer because shopper is a more "comfortable" word for me(and probably most native English speakers) than "the purchaser."