You have to clear out her stock later in the game, trying to clear the shop early doesn't net you the achievement.
I got it when reaching the final campfire in the glass palace, after buying everything new items showed up but I already had the achievement. The campfire is on the second floor of the upper level. If you explore thoroughly and fight everyone, you should have more than enough gold and sellable items to get this.
If you want to get it and keep your gold for whatever reason, make a save, buy out whatever you can, sell them back to her, continue buying. (If not enough gold, sell all your stock, even your equipped items by unequipping then selling). Obviously reload your save after you get it!
This should be easy to get after the final boss so no rush.
Edit: changed information that said I thought you could get it earlier by clearing her shop. Looks like it has to be late game for it to unlock. Thanks Mr Spineless and Devious Jedi for testing it out earlier in the game.