It is not advisable to buy from any place without knowing ahead of time what their return policy is.
If the policy is stated clearly at Pictureline or on their web site, then there is no reason for complaint.
Checking Pictureline's web site I easily find their return policies by scrolling down to the bottom of the home page.

The link to their return policies: http://store.pictureline.com/returnpolicy

and specifically for opened and/or used items it says, "OPENED OR USED PRODUCT RETURNS: (back to top)
At pictureline, we pride ourselves on the quality and consistency of the product and service we provide our customers. We stand by the fact that we sell only new, unopened, unused products that we order directly from the manufacturer. Our customers expect, and will always receive, the highest quality product when ordering with pictureline. Therefore, we are unable to accept returns on opened or used merchandise. If you should have any further questions regarding this policy, please contact Sasha at pictureline.com at 1-800-748-8200"

I do not know if the TC you purchased works well with the 70-200. If it doesn't then some research should have been done before purchasing. If it should work, then Pictureline should return it to Nikon for you and let you try another one or wait for reply from Nikon.
A quick google and I do see that it is compatible with that lens.
We do not know if user error is involved.