[glamisdude]

Unless they sourced the diff I don't know why they would pay. They did not have to take it apart to install it. My guess is all they may have done is remove the cover to inspect (if that). They probably just mounted it in the truck bolted the CVs on and filled it with fluid.
Sounds like it may have been missing a clip from the get go and the ebay seller is on the hook.

Most likely, you'll have to front the cost.