There is no hidden cap when it comes to selling items. The difficulty modifies that value of items sold, as stated on the game rules screen.
The mercantile modifies the "merchant's reaction" which modifies the price of the items you're buying and is capped at 115% min (regardless of difficulty).
What seems to be happening is that all the merchants have their base reaction set to 2.0, so it only takes 85 skill (85 * 0.01) to cap it out.
So, we'll need to up these numbers. These are rookie numbers.