Selling arms to Iran, not only will anger the US, but also other countries in the region, countries like Israel, Saudi Arabia, UAE, maybe even Turkey and Egypt, and it could spill over to Islamic world. Iran is unpopular in middle east, and in islamic world in general. China could end up in a Sunni VS Shiites conflict. Not to mention China has healthy relationship with Israel. There are costs to calculate and it seems to me the cost of selling arms to Iran far outweigh whatever gain. Of course China should keep doing business with Iran, after all many countries secretly did.

Selling arms to Venezuela maybe a better option, but the problem is Venezuela is a mismanaged nation, with so many unemployment and poverty, selling arms to Venezuela maybe will hit Chinese reputation in Venezuela in the long term. Just like how the US supported Marcos in Philippine in the past, leaving a thorn in the future relationship between the Philippinos and the US.

In term of alliance, China is indeed far beneath the US. For one thing, China didn't really believe in the benefit of alliance, which is a fair point, alliance is only as strong as the strength of the leader of the alliance, just look at the US today when some of its allies begin to ditch the US when they saw no benefit.

Secondly, the US has built a very successful coalition of wealthy and powerful nations. In every region of the world, the US is building alliance with the major nations in that region. It is like a match where the US picked all the best players, leaving substitute players for others, LOL. If China want to build a powerful alliance just like the US has, it will be time consuming, very expensive, and no guarantee of success. I think this is what China aim with OBOR. China can also try to steal the US ally, but it is very difficult thing to do, without matching the condition the US gave its allies. Like fully opening the market for them. Even with that, at best you can turn them slightly neutral. And as we see in Australia case, it is a failure.

But not everything is lost. Thank god the US made a huge blunder turning the Russians into enemy. And as the US strength begin to decline, the alliance also weaken significantly. Then there is trump.