定価 and 価格: a list price and the selling price
Distinguish list price, selling price, and purchase timing, without applying a later price change to a past payment.
An established reference price
定価, read ていか, means an established price for an item. この本の定価は千五百円です。 says that the book's list price is 1,500 yen.
定価で購入しました。 says that the item was bought at the list price. Here the actual purchase price is explicitly identified as 定価.
The price of an item in a transaction
価格, read かかく, means the price at which an item is traded. 販売価格は店によって違います。 says that selling prices vary by shop.
定価より低い価格で販売しています。 says that the item is being sold below its list price. In this notice, the list price and selling price differ.
Comparing prices at different times
購入したあとで価格が下がりました。 says that the price fell after purchase. A later lower price does not change the amount previously paid.
価格を確認してから購入します。 says that the speaker will check the price before purchasing. Do not present checking and buying as actions already completed.
Are the selling price and list price equal in 定価より低い価格で販売しています。?
Show the answer and why
No. The actual selling price is stated to be lower than the list price.