There are plenty of cheap step down 90-260 V -> 5 V / 5 watt supplies that only cost a couple of bucks. A crappy "modified sine wave" inverter with non existent safety ratings doesn't actually have to be much more expensive. If it only produces DC as indicated by the suggestion to rotate the plug there is no reason it couldn't be even cheaper. A non-isolated, unregulated or poorly regulated boost converter could be made for next to nothing.
Also, this is ebay. There is no reason to think it is manufactured with the intent to sell it for GBP 1. It could be overproduction of a custom part that someone decided to dump on ebay. If it really produces unregulated DC I have a hard time imagining it being produced as a stand alone product in the first place. This has all the signs of a last minute bodge by someone who needed 12 volt compatibility for a specific product on the cheap. I could be wrong and this was produced for direct sale to consumers but never sold because it turns out there are things so crappy nobody will buy them (except of course on ebay). Or it could be stolen. There are plenty of products on ebay that are being sold for less than the cost to produce them, and often even less than the scrap value.