Thank you for the input, everyone. Here is the message I decided to send off:
"Good afternoon, XYZ. I hope this message finds you well
. I had a thought this afternoon that I wanted to convey to you:
If it is possible, would you be interested in contacting the buyer of the XYZ and forwarding my information to him? I am prepared to make him a generous offer on the card so that both parties could leave happy. I am also willing to send you 15% as a thank-you for putting me in touch with him (I can send you the 15% of your sale of the card right away, or 15% of the possible deal if it goes through).
I understand if you are not comfortable doing this, but thought it may be worth a try as this card is important to me.
Thank you for your time
-NJ"
I upped the percent in hopes of sweetening the deal for all parties involved. Fingers crossed!