It kind of depends on how much you want to drink from the same well. However, taking a sip here and there may be enough to quench your thirst.
I've had people send me long lists of all sorts of parts they wanted and I used to spend a lot of time pricing that stuff only to find they only bought a couple of pieces that I found at substantial discounts. After creaming off the best prices, they moved on to the next lowest priced place until they filled their order. If it was something they couldn't find and I had it priced on the original order, I'd hear back from them a couple of months later asking if I still had the same part. I'd tell them yes, but since the part ( used ) was no longer part of a package price, it was going to be more. Annoyed, they'd hang up only to call back a week later still trying to beat me over the head about my price. This was typical 20 years ago - I haven't seen this in a long time. And no, these people were NOT from America......... You see, there's more to the price than just the price.
Internet sellers can easily handle large volumes of shoppers because they have web sites where you click on to what you want and put it in your shopping cart. And, I'm sure this works well for buyer and seller of new parts with pictures. After all, 90% of this stuff is all the same parts from the same sources. This is a smaller market than you might imagine. For MB, it is without question, their most lucrative division.
As long as you know your car and how it works shopping on line can work for you. If you're not that sure about things then I would advise talking to a real voice. When I first started selling parts over 30 years ago everyone called for what they wanted and you could cultivate a sort of friendship with people. Now we have the internet.