To make that secure from both sides the escrow must handle both the money and the modules which means double shipping costs. MG is in Germany, so that would not work in the US. We don't have a stockroom and I think people would not pay for the additional fees. They are to high in relation to the cost of most used modules.
May be there is a third party service users can use?

Beep, Bopp, Bleep: info@modulargrid.net