Of course these countries have massive profits from being the only ones trading with Russia
Remember the sanctions are a partisan initiative, not a UN resolution. Most world states have not imposed any sanctions on Russia (here). Zhang Jun, China's ambassador to the UN, describes the sanctions as follows:
Unilateral sanctions indiscriminately imposed by the United States and other countries outside the council mandate are entirely for maintaining their hegemony, technology, gold monopoly, and ideology.”
And while there is certainly extra profit from trading with Russia when it is under US/EU sanctions - I'm not sure the profits are that "massive".
also have a monopoly on selling products such as cars, phones, computers, etc to Russia.
Since most world states don't sanction Russia, that is not the case. Also, remember that states and private corporations operating in those states are not the same thing. In China there is perhaps more of a symbiosis, but in India somewhat less so.
So the real-politic answer to why they are not participating to the sanctions seems fairly obvious.
It is fairly obvious, but it's not the massive profit potential you suggest. The obvious reason is that, well, China has no reason to sanction.
You see, sanctions are a rather extreme act in international politics - and often a surrogate to, or a supporting measure of, conducting wars. The US and its allies, on one side, and the USSR on the other side, had imposed lots of sanctions as part of their cold war; but other than that, on the world stage, many-state sanctions are rare, and even then mostly limited to arms/military equipment. On this Wikipedia page you'll find that the prominent examples from recent years are UN resolutions against Libya, Somalia, Apartheid South Africa and a few other cases.
Add to that the fact it's doubtful that sanctions against Russia would be helpful to the situation in Ukraine. Now, perhaps you believe they would be helpful; but many, or most, outside of NATO do not share that belief, and the leadership of China doesn't share that belief, as evidenced by the quote above. Chinese President Xi put it rather bluntly himself, it seems, last year:
[Xi] also took aim at Western sanctions, saying such penalties were a “double-edged sword” that weaponized the global economy and would “bring harm to the people of the world.”
Instead countries should “embrace solidarity and coordination,” he said, while also touting China’s new development and security initiatives as blueprints.
This rhetoric has recently been followed up by diplomatic efforts on Xi's part to mediate between Russia and Ukraine; such an initiative naturally agrees better with a neutral stance of the prospective mediator state.
(By the way - one could argue that China might have better helped by doing the opposite: Threatening to sanction NATO countries. I'm pretty sure there would have been multi-lateral talks for settling the Ukraine question pretty quickly if that were to happen... albeit at great economic cost to the whole world.)
But what is the political justification for this? ... is there a politically correct reason for helping Russia to invade into Ukraine?
China's position is neutral. It is not "helping Russia to invade". Yes, it could have - in theory - acted staunchly to block the invasion, but that does not validate your description of the matter.