Option to place purchase offers.

One feature that could potentially help boost sales a bit would be the ability to post a "buy offer," which would allow people who have the card in their inventory to see the available offers and potentially respond to them.

Example: I have a card in my inventory that is not—or is no longer—for sale (no active listing).
Someone looking for this card submits an offer indicating they want to buy it for xx€ (offer excludes shipping).

People who have that card in their inventory can then see that there’s a potential buyer and can respond to the request by indicating the total cost (price + shipping).

The buyer then confirms the response (+ payment).

I think this could be useful for “somewhat expensive” cards (such as numbered cards or those with specific rarities) that some collectors might sell individually if an attractive offer is made.

Additional note: From the inventory, users could view purchase offers for a card in addition to sale offers.