Sales gradually decreased.
The DVD of the Hollywood movie in which a popular American actor played the main character has sold eight million copies in Japan alone.
Sales have decreased rapidly because of that sexist advertisement.
The selling that you've raised (上げ) is the amount sold or number of sales. Think of a graph going upward, where it's rising as you sell more things.
The reading for 売 is the one you learned with 売る. The reading for 上げ is the same as the reading for 上げる, a word you learned previously. If you know those two separate things, you should be able to combine them into one awesome vocab thing.