This kind of trade with the enemy is actually very common throughout history. The UK traded scrap metal with Nazi Germany, the US purchased platinum and uranium from the Soviet Union. There are many other examples going even further. It usually takes place through third party intermediaries and isn't as contradictory as you might think. Sanctions aren't typically intended to completely isolate a country and often doing so is counter productive anyway. Not only does doing so hurt civilian populations but it can force even more extreme measures. Japan's expansion of the war in the Pacific as a result of the US withholding oil is a perfect example. Also sanctions can and often do have blowback.

The sanctions against Russia and particularly the limitations established on the sales price ceiling of Russian oil and gas exports are a moderately effective means of exerting leverage. And while not perfectly enforceable the price ceiling does keep the global pice of oil in check. Which in this time of extreme inflation is very desirable. Any time you have a price differential there will always be profiteers there to step in unfortunately its kind of unavoidable. However barring better options it better than nothing.