Numerous dealers in the UK have eBay shops where they sell a multitude of parts at quite attractive prices ; the situation is much better in Germany , and also don't overlook places like Poland , Lithuania and Latvia where so many genuine parts can be obtained at very reasonable prices .The tricky part is that in order to get the parts directly from MB with trade discount, you'll need to be VAT Registered which will increase the cost of your services to your customers.......
But, of course, you can buy the parts from MB dealership online as retail at a reasonable price.
(BTW, MB prohibits non-franchised garages who buy parts directly from them, from selling-on the boxed parts - I.e. the parts can only be sold if fitted to the customer's car by the garage and labour is charged in addition to the price of the part)
Having access to EPC and WIS can be invaluable , and for newer cars the iCarsoft will not only interrogate the car for faults but will also list up the M-B part numbers for whatever is required - not much use for may of my cars though since they predate OBD and I have to use the manual to interpret blink codes , which only really started in the seventies .