Mindy is copying Rex, but when I hover over her portrait, it shows she always has the maximum DPS buff from "Trophy". The number never changes or resets, which means she isn't turning the trophies in for Black Market buffs. Looking at each individual buff over the sprite's head, she's only got "Human Condition", "Boxed In", "Trophy (Mimic)", "Exotic Markets (Mimic)", and "Creative Trophies (Mimic)", but Black Market is not being mimicked, so it's buffs are not being applied by Mindy.

Before posting the earlier comment, I removed all crusaders except Mindy and Rex, so I could monitor their buffs by themselves. Like I mentioned before, the circles above each crusader's heads show each individual buff they are giving or receiving, and I listed every effect Mindy had above her head. "Black Market" was not one of them, so, for me, Mindy was not even mimicking Black Market in the first place to be ABLE to sell any trophies. For clarification...

-Yes, Rex had "Black Market" bought and active
-Yes, Rex's "Black Market" was behaving properly
-Yes, Rex's "Black Market" buffs were being applied

So Rex, himself, works. Mindy just won't mimic him and/or sell trophies. Her "Currently Copying" list shows she is copying "Black Market", but it does not appear above her head as one of the circles, and trophies are being collected, but not sold.