I picked up this achievement in Phase 6 Standby, just by selling everything (I saved first obviously). You don't even need to sell one at a time. Just go down the list hitting
until the amount I was selling for (bottom right of the screen) hit 200k.
To save time, I also recommend using the Fashionista and Exchange achievements to accumulate stuff to sell for 3,000 or 6,000 kin.
NOTE: I've heard it's also cumulative but going that route can takes longer.