To put things in perspective, $400 is the equivalent of 3 full months working on a minimum wage here in Brazil. I understand that people are entitled to do whatever the hell they want with their own money, but to be entirely honest with you this can't be right. The only two reasonable explanations that I've got for this are:
a) The buyer is a millionaire or a very wealthy person that considers the keycap as being a piece of art;
b) The buyer doesn't have self control and decided to burn some late year spare money that he had;
Outcome a) is absolutely understandable and I believe we all agree with it. Rich people spend much more money in much simpler stuff than a crafted keycap, it happens all the time. But the outcome b) is the one that worries me a bit, because impulsive buys like this can be considered just as dangerous as betting or gambling. Sooner or later you'll be swamped in debt and will go broke. And I honestly believe that such action is an indicator of obsession and other psychological disorders. If somehow the buyer eventually gets to read this, I recommend you to seek for adequate help my friend.