Hi Josh,
I wrote an answer, and when I clicked post, I got the blue circle of death and my response disappeared, so I am rewriting my answer. I say that just in case my previous response suddenly appears out of the ether.
While I have not faced this exact situation, I had a remodeling client ask if I would consider amending my terms and conditions to allow them to provide copies of my drawings to a hypothetical future buyer. I consulted my attorney, and he said he thought that type of use was already in the scope of the license. To satisfy the client, he suggested amending the intellectual property section of my terms and conditions to include this language:
Expressly conditional on payment of all fees and other charges owed under this Contract, Studio Z grants to the Owner a perpetual, non-exclusive license to reproduce drawings and specifications as may be needed for purposes of maintenance and other ownership requirements for the completed project. In addition, that license shall permit the Owner to provide copies of Studio Z intellectual property to potential buyers of the property where the project is contemplated provided that any such copies are unaltered and include the full Studio Z title blocks, identifying and contact information, and copyright insignia as appear on the original versions.
I would not allow a client to "sell" my drawings, as they don't own them. However, allowing them to give copies of your drawings, including your contact information and copyright insignia, would be a nice gesture, and might even earn you another client.
Definitely consult an attorney.
Good luck!
------------------------------
Dawn Zuber, FAIA
Studio Z Architecture
Plymouth, MI
------------------------------