Selling the boxes isn't illegal. Streaming through your internet is though.
To be honest, we've seen this for years from dodgy sky boxes to dodgy chipped cable boxes. They were all stopped and all were soon replaced with something else. It'll be the same if they ever stop the Kodi boxes, which I guess is a few years away yet!
You can download the kodi programme to your laptop, firestick or number of other devices, so there's not actually any need for the box.
One thing is certain. I won't be paying the £135 that my mate pays for Sky when I can get the same for free....even if it is usually a minute later!