On the other hand, Russia was rumored to have been secretly buying Iranian oil previously and then reselling it as their own to European countries. Why couldn't they do it all over again?
After all, the US has no access to the Caspian Sea to check that trade (unlike the Persian Gulf)
This is the first time I am hearing about it. How can Iran smuggle oil to Russia, when dozens of American military satellites are scanning every square inch of the Russo-Iranian border? Also, I don't think that Russia will take the risk of dishonoring the UN sanctions.
I would imagine Iran moves their oil via ships. The largest importers of Iranian oil are China (22%), Japan (14%), India (13%), and the combined EU (18%) with Italy and Spain comprising the largest there, followed by, to a much smaller extent, France, Germany, and the UK. This information was from 2011, though. Plenty could have changed since then, but I would generally expect the ratios to be in the same ballpark. Just because the US slaps sanctions on Iran doesn't mean everyone else is going to follow. Each country is going to pursue its own foreign policy as it sees it to be in its best interest.