Granted I wouldn't have noticed this being possible without training wheels in the form of leap of faith. Still, it is possible to garner purple coins at a fast rate with just basic buffs as well as good twitch and timing.

http://www.youtube.com/watch?v=RDApu0PhxIo

Instructions:
*Jump to get above the final horizontal slash on the ramp opposite the coins
*Turn around as you go down
*Go forward as you reach the level part of the ramp and are facing the coins
! Jump at the very edge of the ramp
! Turn around quickly as soon as you touch the edge of the purple coin

Difficult, but doable.