Back in the day......
There were many local independent gas stations with lower prices than the majors. My father had a Texaco and a Sunoco station, we lived not far from New Haven CT where gas was offloaded from tankers. If you want to believe him, here is his story.
Beware of stations that are not associated with a refinery. If there is no refinery, they must purchase gas from a source affiliated with a refinery. He claimed that sometimes gas would be auctioned off in lots, either surplus, or from a bankruptcy, or whatever. But more likely it was gas compromised in some way, such as by seawater. So the risk was you might be getting Shell or Sunoco on the cheap, until you hit the bottom of the lot.
These days we don't see much of this, not sure exactly why. I'd like to think technology minimizes contamination and can filter out what is contaminated. I'd like to believe...
