Originally Posted by Milton Keynes

I wonder if they were parts swappers? Are the shifters the exact same ones you sold them? In the radio hobby world some times "parts swappers" with a broken radio will buy a similar radio from someone online, swap out the bad part with the good part from the new radio, then contact the seller and claim the radio didn't work upon arrival and demand a refund. They get the refund, send the now-broken radio back to the seller and use their repaired radio. I was wondering if maybe these two hatched a scheme to swap out their broken parts with good parts, but it would take some knowing about bicycle parts and getting the exact same parts they wanted to swap.

Still, claiming "these parts don't work" and then giving them back to the OP in broken condition ought to negate getting their money back.