Targeting enemie's fiefs, buying all S&D in a small region and selling everything at +0/50% bonus not far away in a single day/night is highly doable and highly destructive to one's economy, especially if repeated on a larger scale (geographically, or periodically, or both).
Frank idea isnt ridiculous, its very effective and very easy to pull, with silver being unlootable and some luck.
Additionnally, with crime being as it is, the prosperity must not be too high, thus the decreased overall S&D production is even easier to completely rip off.