At the beginning of the game, I was hording Ripper Burls and throwing knives in my stash to sell all at once for this achievement.
Don't do that.Towards the end of the game, you'll start finding wands worth 2,500, rings worth 5,000, and weapons with a value in-between. Keep in mind, whatever value is shown, Aelita will give you 50% - so you'll need two items worth 5,000 for example. The red chests have the best loot, and you'll be drowning in money and valuable items by the endgame.
NOTE: Aelita only ever had <1,500 silver at a time for me. You will likely have to purchase something for >5,000 silver, unlocking this:
Then, she'll have enough silver to buy from you.