Others can just go to the store

Other people going to the store doesn't stop scalpers from clearing the shelves. If they restock overnight, scalpers can and do wait for stores to open just so they can clear them out.

Logic implies that the prices are being compared to other stores.

Imagine this scenario. Private grocery store (PGS) sells bread for $5 a loaf. Municipal grocery store (MGS) sells bread for $3.50. PGS decides to cut their profits margins (citation needed) and sell at a competitive $3.50. MSG sells bread at $2.75 (after all this is 70% of 3.50). This repeats until the PGS goes out of business and the operating cost of MSG is much higher than expected.