Just what could the sensitive IP be that these units in question contain ?
Nothing. In my case it's just a humble 34401A. Only thing valuable for them here are serial numbers on the back.
And the serial numbers may be enough to make it matter.
Here's how the law really shakes out on this. If the original seller and original buyer contractually agreed to dispose of some object in some specific way, and that didn't happen, the problem rests with that original buyer. THEY are the ones who subsequently sold/discarded/etc. the item in question differently that contractually required. The only way this would be different in normal commerce is if the original buyer somehow made the next possessor a party to the original contract terms. An example of this might be a subcontractor on a project; say McDonnell Douglas has the top-level contract but they sub out the avionics to Garmin. Garmin might have a legitimate need for the "special" test equipment, so McDD would include provisions in their subcontract with Garmin making it very clear to Garmin that the equipment either had to be returned or provably disposed in some traceable manner, so McDD could then prove to their customer that the terms of the original contract had been followed.
If the above didn't occur, then the responsibility for compliance rests with the original party to the original contract. The problem for THEM is that, if the equipment in question was sold outside the terms of the original contract, they still possess the obligation to perform. And if that performance is linked to specific serial numbers, then they literally have to get the original devices back or be able to conclusively prove that the equipment was lost or destroyed beyond their ability to recover it (accidents do happen). They can't just shrug their shoulders, and they can't get someone like Outback6 or the eBay purchaser to perjure themselves and say "it's lost". They're stuck with the liability and have to fix it.
On the eBay buyer's side: If the equipment was stolen and then sold on eBay, the eBay buyer may be out of luck legally. You have no right to stolen property since the property rights were not properly assigned by the original buyer. The eBay buyer might be able to recover their money from the eBay seller, but that is entirely separate from the question of possession and legal ownership of the item itself.
If the original buyer mistakenly transferred the item to the eBay seller, then the eBay buyer could be in a weird place. You could argue that there is a paper trail proving transfer of ownership, but remember that you cannot assign rights that you do not possess. If the original buyer did not have the legal right to transfer ownership of the item, the fact that they wrote and signed a document saying otherwise doesn't mean it's legitimate. They are still on the hook for compliance with the original agreement unless the other party to the original agreement releases them from that requirement. Doesn't sound like that's the case here!