...or a customs declaration. If I want to send parcels from Germany to the USA, I have to stick the customs declaration with the weight, quantity and value visibly on the outside of the parcel! This will certainly not be any different for other countries, as this is a requirement of US customs.
I would monitor ebay, craigslist and other second hand plattforms. The transformers are not easy to sell despite their high value, as they are very special.
Do they have known serial numbers or other unique features that you can use to prove they are yours?