Selling a stack at a time only pays you for 19 of the 20. (Item-specific! *NOT* all)
 

Summary: Selling a stack at a time only pays you for 19 of the 20, while selling 1 at a time sells all 20 for the stated price. This is item-specific and does NOT apply to all items in the game.

Repro steps

1. Put all money in bank
2. Remove 1 gold from bank
3. Visit merchant who sells unlimited items that cost 5 copper each
4. Buy a stack for 1 gold
5. Ask merchant what they'll give you to rebuy.
6. In case of most items it is less than the 5 copper.
7. In case of some items it is exactly 5 copper (same as purchase price).
8. Sell 1 at a time (control-Click the sell button). You receive 5 copper per item. Selling all 20 one at a time gets you entire 1 gold back.
9. Buy a stack for 1 gold total. You should now have 20 items and 0 money.
10. Shift-click to sell the entire stack at once.

Expected result:

1. Game text tells you that you receive 1 gold for the stack.
2. You receive 1 gold for the stack.

Actual result:

1. Game text tells you that you receive 1 gold for the stack.
2. You receive 9 silver 5 copper for the stack.

Testing with fishing bait and other small unlimited items. Results vary depending on what item.

did you test it with food/drink and comsume 1 of the 20 before the transaction was complete?

No, that's an interesting idea.