My 2¢:
There's a big difference between saying, "I'm not happy because of [...] I'd like to return it for a refund."
and "I'm not happy because of [...] and tried to fix it but couldn't, I'd like to return it..."
Not saying this is what happened here, but "both parties not happy = full refund" isn't so white and black.
Actually it is - if you don't treat the exchange as a $25-an-year-money-making-enterprise. Buyers remorse, sure, happens - something that looks very attractive in photos may not look and feel that great in hand. Just undo the transaction and be happy. Both parties are wise for a shipping fee. If losing a few $$ to undo a transaction is an issue for you, then you probably have bigger problems to solve before trying the exchange.
As for this case, calling "pivot tightening" as "tampering" is like calling a brand new shoe pair "used" because a potential buyer tried it on and walked a few steps in a shoe shop. The case is black and white, if the buyer and seller did what they say they did (at least appears the buyer is stand up guy) - simple refund and enjoy other parts of the forum!