What do you mean?
I mean that even if one country (say, Saudi Arabia or Iran) doesn't sell oil to another country (say, the United States) but they do sell to another country, (say, China or Russia) then the end result is unchanged. The price of oil stays relatively stable and the sanctioned country still makes about the same amount of money from their oil.
Part of the problem here is that if you successfully quarantine an oil exporter, there is instantly a drop in global supply and prices instantly skyrocket.
The United States is currently begging Saudi Arabia to increase their oil exports (generally, not specifically to the U.S.) in an attempt to directly compensate for the loss of Iranian exports in order to keep oil prices relatively stable.
If the U.S. stopped buying Saudi oil, it would have no effect on Saudi Arabia's income unless all major oil importers joined in.
It's a gigantic shell game. If you buy oil from ANYONE in a global market, then you are supporting the price of oil which benefits EVERYONE selling oil, even if you are not buying from a particular seller.
Not to mention that embargoes and economic sanctions violate FREE MARKET principles.
If Saudi Arabia AND Iran both stopped selling oil, we'd be staring down the barrel of $10.00+ a gallon at the pump.
Imagining you can "just buy more oil from Canada" is a pipe-dream.