I'm not saying that design documentation is not useful or needed, but it should be regarded as the first draft of the final product and not a bible. Instead of trying to pin point every detail down on the design phase, try to document the core functionality and the purpose of the system. Then in the implementation phase it's essential to have a product owner,or owners if it's a very large and complicated system, that understand the functionality and take responsibility of the use cases on both the buyers side and on the suppliers side. This way the details can be figured out while both the project teams and the customers understanding of the project increases.

We have implemented this typoe of procedure in several projects in the sofware company where we work and we have been very pleased with the results.