This is the risk countries take on when they willingly and openly break treaties they entered into. Particularly those that maintain local, regional or world peace.
Under the sanctions, no West based entities can buy.
The article doesn't mention that Putin threatened to nationalize Western property in Russia, which presumably would include BP's 19.75% stake in Rosneft. So it's not surprising they want to sell while they can.
Question is: who in their right mind wants to buy?