A quick note: Based on a bit of googling, most generic corner gas stations have an underground tank of 10,000 to 15,000 gallons.
If you think about that, it's only 1000 to 1500 fill-ups of 10 gallons per customer. Now think about how many people are at a station at any given time, and how long it takes for them to do so. Even in the case of the 15,000 gallon tank, that's a bit more than 200 customers per day, which comes down to only one customer every few minutes (which fits with what I see at the pump).
Given that, it seems that the gas station probably exhausts most of its reserve inside of a week, two weeks at most, so they're hardly hoarding gas for months on end to make obscene profits.