the seller did NOT mention I had to sign anything on his ebay page, so it was technically a breach of contract on their part
That is an interesting point: is it incumbent upon the seller to make the buyer aware of any applicable laws pertaining to the item being sold.
Here in the UK there are a lot of laws about the ownership of motor vehicles. For example, it's compulsory to have insurance at all times, there is an annual ownership tax, and once a car is more than a couple of years old it must pass an annual safety inspection. Failing to comply with these requirements can lead to prosecution and confiscation of the vehicle.
If I buy a used car on eBay I don't expect the seller to tell me this. The onus is on me to know, and to make the necessary arrangements. Would I consider it a breach of contract because the vendor had failed to mention it? No.
I know that thermal cameras are a little different in that the vendor can be held liable if the buyer does something silly, but expecting a buyer to sign a suitable transfer of ownership paper so that the vendor is covered. In my view it's not an unreasonable thing to request or require. Back to motor cars for a moment: part of the transfer of ownership here is a little piece of paper the vendor sends to the relevant authority to notify the change of ownership and transfer legal responsibility for the vehicle to the new owner. Without that, if the buyer were to drive the car without paying the necessary taxes then the vendor could be liable.
Funny old world, isn't it.
Getting back to the original point then, it isn't unreasonable for the eBay vendor to want some proof of sale and while neglecting to state that on the listing may have been impolite, I don't see it as a deal-breaking breach of contract. The law's the law, whether you agree with it or not.