Sounds to me that you may have a couple that are going to pull a swap on you, send you their broken ones and claim their payment back.
When you are in the business of selling anything, and accepting payments like PayPal or credit cards in general, you should always factor a fraudulent % into your bottom line.
I actually operate what I call a fraud account, and I put 10% away of my gross sales into that account, and then use it to pay out on these type of situations.
I find I get hit at around 3% per annum personally, so my fraud account remains very well topped up and I treat myself to a cash bonus at the end of the financial year by trimming some cash out of it.
I'm quite lucky with that sort of rate, but I do everything in my power to keep it as low as I possibly can. I know of others that get stung around 10% on per annum on charge backs.
You can catalogue all parts serial numbers, and keep that log completely up to date with all the relevant purchase details. That won't stop the charge back of course, but if someone does pull a swap then you have the record of proof.
What good is it?
Well at the end of the day, depends how serious the police treat it, and that all depends where you live, manpower, crime rate, you name it!
What I would do is to then list the fact that all parts are tested and make a serious point to inform potential buyers that I also have their serial numbers categorized to prevent fraudulent swaps.
That may have the effect of moving the crooks past your listings if they think there will be problems trying to screw you over, and they may just try to find an easier target.
The best thing you can do is to try and collect payment by more secure methods, direct transfer for example. That will take the chance of charge backs out of play completely.
Even offer a small discount if people choose to pay via bank transfer as that may encourage them to close the deal on terms that are much safer for you as the seller.
Always use the feedback system when dealing with ebay, it can help you to find potentially problematic members by low feedback scores and such, not fool proof, but you need to use as many tools as you can, and put as many procedures in place to protect yourself.