"how much should the price $ you paid for/acquired a card influence your flexibility on price when selling it?"
None. To think otherwise is the
Sunk Cost Fallacy. The basic idea is that money that you've spent is gone, and irrelevant to the current value of something. If you paid $X for something, and it's now worth $(X - Y), that you paid more for it than its currently worth doesn't make a difference to what its currently worth.
The inverse of all this is that the fact that you got a deal on a card shouldn't lead you to sell it for less than you would otherwise.
It's probably not a good idea to think of yourself as having made a loss or a profit on any given card. Since the money that you used to buy the card is gone, the only question that you face is "how can I maximize what this card sells for now?". And that price might end up being higher or lower than you bought it for.
Now, it probably
is a good idea to ask if you're making money selling cards in
general (if not, you might want to stop), but it doesn't make sense on any given transaction.
Of course, people fall for the sunk cost fallacy all the time. So it wouldn't surprise me if folks do take this into consideration. But the question was about what one ought to do, and it's not rational to take retrospective costs into consideration when making decisions.