Often they lie to producers to lower prices then lie to consumers by selling these deal at the usual rate (or faking a false discount week for appearances and then put back products back on the normal shelves).

As other said, economic and information asymmetry is not good.

Honestly .. aggregators should be a national thing, there's almost no competition to be had there. Kinda like internet, it should be way more neutral, except maybe on safety check (again, something to nationalize).