Don’t know how all main dealers work but in ours service, parts and sales are financially independent of each other. Service department has to ‘buy’ it’s parts from the parts department at full retail price, if we, the service department, discounted the parts we would make a loss, as a customer you could buy your parts direct from the parts department and possibly get a discount. The service department makes no profit from parts, it’s profit comes from selling labour. Fred in the shed will buy the parts at trade price from our parts department and may not charge their customers full retail for it.