I think Allegro lowers prices because it forces sellers into common arena where they have no choice but to compete with each other. It's usually cheaper to buy stuff on Allegro than on dedicated e-commerce site.
But think about it - when was the last time you actually got a good comparison in this market between allegro and a dedicated e-commerce site? For many product categories the latter almost doesn't even exist anymore.
Maybe tipping point is ahead of us. But Allegro already has 20mln unique users monthly (in a country of 36mln people) and twice as many as next largest, AliExpress.
But think about it - when was the last time you actually got a good comparison in this market between allegro and a dedicated e-commerce site? For many product categories the latter almost doesn't even exist anymore.