I don't really understand, this seems far from anonymous. You as a seller can log/save the PUK and card number or whatsoever, so you are the single point of failure which means that for example if you work with some goverment, these sim cards can be traced hence they aren't anonymous.

If someone really wants an anonymous sim card, don't buy one online. Go to some foreign country (if it's possible) or to some far away located shop and buy the sim card there. In most countries you can register these cards with some random information. There are countries like Germany where it is now required to identify yourself in order to activate a prepaid sim.