I may be thick as pigshit. But can someone explain to me how the West can decide what price Russian oil sells for?

I know they can pledge amongst themselves to not buy Russian oil for more than $XX a barrel. But how does that stop Russia from saying 'OK then. We'll sell it to someone else who WILL pay what we want'. It's not like there aren't plenty of other markets for oil, outside of EU/USA.

I know the article mentions that the shipping & insurance companies, which are mostly based in Europe, could refuse to ship or insure cargos of Russian oil, unless it was being sold at below a western imposed cap. But, again, it seems a bit odd that there aren't lots of other shipping & insurance companies outside of EU/USA which would be more than happy to scoop up some nice fat juicy contracts or insurance premiums for shipping & insuring cargoes of Russian oil.