Not totally true.
They subsidize now but they are setting up the infrastructure.
Eventually they will have the economies of scale so they will still retain certain pricing power and negotiation power with collaborators.
They earn from sellers or other collaborators behind the scene to fund the ops. They can stay afloat from fees, charges and others.
It's not a simple buy sell business. Technically this is reducing one more layer, it's China to Singapore direct. So pdd and taobao will each have a clear advantage.
