Price alert of product

Envato Customers Purchases and Subscriptions
1

Is there a way to be alerted if a product goes on sale or is reduceded?

2

yes

3

How can one be alerted if a product goes on sale or is reduced?

4

If you mean it for envato then answer is no. There is no such api/features to know it.

But envato sometimes send newsletter to the subscriber where you will come to know onsale products links. So, you can subscribe if already not done!