If a buyer states that they received something that's not as described and I have evidence to the contrary, the first thing I do is send a message that seeks to rule out some possibilities as a formality (eg depending on what the buyer is claiming, I ask if they bought a similar item from someone else and have me mixed up with that seller, if missing items or damage, I also ask for condition of the packaging on arrival, and whether there was evidence of packaging tampering, particularly if the package was re-sealed by Aus Post, and advise that any images they can provide will assist in the fastest, most appropriate resolution possible).
I don't say anything that would suggest "I think you're trying to scam me", just that there seems to be a mistake somewhere... I refer to the records I've kept of the transaction which indicates the package contained exactly what it was supposed to in the condition as described, and provide evidence of that ("for their own peace of mind, so that they can verify my statements to be true and that any errors were not intentional on my part"). These records include (where applicable) remaining stock, package weight, and images recorded of the order during the packing process.
So far, almost every buyer who I have responded to in this manner has come back and admitted they made a mistake... I did have one buyer who was hell-bent on getting items for free, I actually just accepted the return because the value was so low, and they sent a registered letter that didn't contain the items I sent them (they did get a refund, but not from me - stoopid PayPal 
).
Now, I think if a buyer tried to insist that what they received wasn't as described and I had more than good enough reason to think they were trying to pull a swifty, I would advise them I will accept the return but that the returned package will be opened in the presence of police witnesses in order to have an official and impartial third party to be able to verify the contents and condition, and that I would pursue all legal avenues if I receive something other than what was sent. It might seem over the top to some, but I think one of the biggest problems sellers who are being scammed face is the common belief many of these dodgy buyers have is that you can't prove any wrongdoing and can't do anything about their actions, and I have experienced enough of that attitude so would rather have as many of them as possible know that's not actually the case at all.
