@jenijensong,
"A customer purchases my item for sale and then switches their old parts with my new parts or puts my new parts on their old item and then says the item was not as described and damaged during shipping".
Given that you posted on the Electronics board, I would have to assume that this is about the turn table you recently sold. There are only a few things a buyer could "switch out" on those units and they would be hard to remove and replace. Chances are if they were going to return the item, they would return a completely unit. You could protect yourself from that by posting a photo of or just the serial number in the description.
If you had received a different unit back, as the serial number would indicate, you would have several options for getting your payment restored, as @stainlessenginecovers posted. Starting with contacting the local police in the buyer's area, and reporting mail fraud to the USPS postal inspectors. The IC3 isn't very helpful in these situations. If they found in your favor you could use the report buyer button in the abusive buyer policy (link below)
https://www.ebay.com/help/policies/rules-policies-buyers/abusive-buyer-policy?id=4374
It is also possible the buyer was right about shipping damage. Those turn tables do require re-adjustment of the linear tracking system, which may also be thrown off by rough handling. Especially, if you did not have the original box the unit came in.
"I don't want a return since I'd be getting old parts on my new item (long story how I found out) but I don't want to keep giving more refunds & losing all my profit since I sold it way cheaper than I should have".
The only way you could find out if you received different parts or a different item back, and be able to prove that you received replaced parts or a different unit, would be to accept the return.
There aren't that many ways you can find out if the buyer has a habit of doing what you think they have done. Their feedback profile, or social media accounts are easy to look up. As for the price you received it, you did get swell more than the trending price for the item, so it might have been worth it to accept the return.
"Ebay helped me resolve the problem but I hope it doesn't happen to anyone else".
Since ebay apparently did side with you, it was because you provided proof of the buyer's activities. That in itself is rare, since 99.9% of the time they side with buyers, when not as described disputes have been filed.
Unfortunately, because you chose not to describe how you got the info that helped you out, your "warning" doesn't do other seller's any good. That info is what would be helpful. People have been returning items with different and even missing parts, their damaged items, and even packages with things like rocks in them, since ebay started and before they had a money back guarantee.
"I didn't take out full insurance on the price".
Insurance, is for a seller's protection only, and shipping companies tend to try and absolve themselves from admitting they caused damage. Still it is better to have it, than not to. Getting buyer's to work with you and/or the shipping agencies isn't always easy either.
"THERE IS NO SUCH THING AS FOOLPROOF, BECAUSE FOOLS ARE SO DARNED INGENIOUS!" (unknown)
