The danger with part-worn tyres is that there are very few checks on the origins of the tires, or the reason why they were removed from the previous vehicle.
In the UK, the lobby group Tyresafe has called for a ban on these things:
https://www.tyresafe.org/media-centre/latest-news/vast-majority-part-worn-tyres-continue-sold-illegally/
The UK National Tyre Dealers' Association is against them:
https://www.ntda.co.uk/useful-links/part-worn-tyres/
Internationally, the Spanish police intercepted a ring of dealers who smuggle these things and sell them without any checks:
http://www.guardiacivil.es/es/prensa/noticias/6573.html
The Ohio State legislature has banned them
https://www.legislature.ohio.gov/legislation/legislation-summary?id=GA132-SB-223
The US tire safety body (and industry lobby group) warns against their use:
https://www.ustires.org/system/files/TISB_45_USTMA.pdf
in the end, it is often less dangerous to buy a cheap tyre made in China or elsewhere that is new (you can get them for £40 or less) with 7mm - 8mm of tread, than to buy a used tyre for which you do not know the provenance for £20 with just 3mm of tread, bearing in mind that the minimum legal tread depth is 1.6mm