Poor customer service

I found a vehicle that I wanted to buy but was told by the salesman that you are only allowed to buy cash or finance through webuycars. I wanted to finance through Investec (whom webuycars do not deal with) due to much better rates, terms&conditions etc, but was not allowed to do that. They were not able to tell me what the difference is in me paying them the cash or Investec simply paying them. I was told by the salesman and also helpdesk that he does not make the rules, which I understand and can't fault him on. Perhaps someone that made the rules can contact me and tell me why they are happy to turn away a customer.