I don't think this subject goes here but anyway...
What would you do if you won a unique card. Let say UR makes a card and there is only one of that card, yours. Ok stats (3* 7/5) Uppers, ability: SOA, automatic Cr, cool art, cool name.
This card is worth 70,000,000 clintz and you get an offer for it... Would you sell the card (knowing you let go a unique card that only you would have) or would you keep it for collecting purposes. If you sold it you could buy all the UR collection (Including DJ Korr Cr, General Cr, Kiki Cr)
I would probably agree with YayoiCR,
the person buying your card would probably have finished the collection already and would just be sitting on a huge pile of cash. Buying the card would result in him finishing the collection and never selling it again thus disabling everyone else from completing.