The Digital Service Book (DSB) will contain records of all services carried out by MB dealers, and also of services carried out by authoritised independent specialists. You should get the seller to provide you with a printout of the DSB before you do anything else.
Then, the DSB printout could be supplemented by either printed receipts from non-sepcialist garages (who do not have access to the MB DSB), and/or by invoices for service items such as engine oil and filters if the previous owner(s) serviced the car DIY.
The potential issue is that smaller traders might not be able to obtain the DSB printout from MB, and if it wasn't provided by the previous owner when trading-in the car (or if it was lost at some point after that), then the car is effectively sold without service history, even if the seller is trying to convince you otherwise.
In terms of value... the gold standard for a 10 years old car is obviously straight 10 services done by an MB dealer. But these are rare (though not entirely unheard of, especially if the previous owner took out the MB Service Care Plan). Typically, you'll see the first 3 services done at an MB dealer (while the car was under warranty), then the next few services by an independent specialist, and possibly the last two services by a non-brand-specific smaller garage. I'd be happy with that.
Now, this is true for the standard cars. With AMG cars and other rare/expensive models, the expectation is for MB dealer servicing throughout, with the only exception being services carried out by respected non-franchised AMG specialists (such as MSL in Birmingham).