I'm sure this surprises no-one who has ever waited for a Black Friday (deal) only to suspect/know that the price is no different than before. It feels especially this way in Europe where retailers have adopted "Black Friday week" or similar purely to encourage additional spending. There is a tool (in Switzerland) I use a lot. TopPreise ( http://toppreise.ch ) scrapes, aggregates and compares prices from retailers but…
In Germany, https://www.billiger.de/ https://www.idealo.de/ and https://geizhals.de/ are just three of the many sites that aggregate prices.
I know Idealo has a price history that goes back one year. The unfortunate parts: they only list price history for new items, and incorrectly listed discounts (refurb instead of new, different model/color, etc.) stay in a product's price history, so there are sometimes price valleys that should not exist.
It would be nice if you could see (on the price history chart) which site/seller had the lowest historical price, since there's more credibility in a listing from Media Markt (probably the largest electronics chain in DE) than from Ebay.