My background is in illustration and fine art where copyright is likely a bit better understood.
When you sell the actual 3D model, you need to first recognize you own the copyright to the design (if it is your original design).
When you sell the model, the client will have the ability to leverage that content in many ways:
- edit it and reuse
- publish it (with or without credit to you)
- develop a variety of 2D representations based on it (could be taken to a 3rd party for rendering)
All these have value, and depend on how valuable your design work is in the market place.
In the advertising illustration field, selling the total copyright is a percentage of the job it was commissioned for - in the 50% range.(100% or more is common too).
You can also sell partial copyright, although that could be difficult to police.
The client often needs to be educated as to why there is a copyright cost at all and they will likely prefer to keep it a secret as to their plans for your model.
Attached general copyright info (Canada).
A guide to copyright - Canada.pdf (316.9 KB)