It's not a matter of trust, it's simply protecting his investment. This isn't loaning someone some gummy bears, this is a Mercedes-Benz.
They know the deal. No matter who they are, it's not their car until you've established some payment agreement with them and paperwork is signed on payment arrangements, or a check in full is received. Then they can do what they want with it.
Until that point, this is your vehicle, you have an interest in it and you can be there to oversee the "inspection." That's not trust, that's basic accountability. If they don't understand that, they don't need to be driving an E-Class.