Some buyers don’t happy with their sellers due to misunderstanding among buyers and sellers. Most important problem is that they both are correct about their understandings but no one want to admit his/her mistake.
If the buyers follow documentations about their projects they can claim that what they need and what their sellers developed.
If buyers are good in communication and able to promote their vision, that will be helpful and no need to give bad ratings to the sellers.
Use pictures, Demos, PDF files, and word documents for your documentations.