I work in a test lab where we used to test seat belt anchorage installations. Lets just say I wouldn’t strap my kids to the seat in your pictures, even if extra / full belts were retro-fitted…..
A proper, safe, M1 tested, 3 point belted seat will be somewhere around £2000. Google RIB and Scotseats as 2 examples.
I’ve looked at some of the seats on ebay etc and the wording is very misleading. If the seat design hasn’t been tested to M1 loads and installed in a manner that could pass the M1 test then it is of dubious legality (i.e. if it came to a claim for an injured passenger your insurance company would probably say it wasn’t fit to be a passenger seat and you aren’t covered).
I don’t care if a company has sold / installed hundreds of seats, if it hasn’t been tested (or real life crashed with some very heavy passengers) then they have ZERO experience or evidence.