So if you stick, a stack of items into the trade window (say 5 lockpicks), and you remove some of the lockpicks from the trade window, but not all from the stack, the value of the trade does not go down, and you get the full value, even though you aren't selling the full stack.

In other words, each lockpick is worth 12 coppers. So 5 of them are worth 60 coppers. If you stick 5 lockpicks in a single stack into a trade window and press the Trade button, you will receive 60 coppers. If, instead, you take 4 lockpicks back out of the trade window and then press the Trade button, you will still get 60 coppers. I tried this with quite a few other items (pelts, horns, potions, herbs, etc), and it's bugged like this with all of them.

Are you sure your total gold is actually increasing so much. I tried this, for example, I put a stack of 5 items (12 cp each) in the sell box and it showed 60 cp trade value. I removed 4 of them and it still showed 60cp in the trade view, and I clicked trade. But when I closed the trade window and checked my gold level in the inventory screen, it had only increased by 12 cp for the 1 item sold.

in my game, it was increasing the amount by the stated value, not the actual value.