How do I discount every product except a few bestsellers?

In Plugins → Bulk Price Update:

1. Set Apply To to Selected Products.
2. Set Match to Exclude — all except these.
3. In Search & Select Products, type at least 2 letters of a name or SKU and click each bestseller to add it.
4. Change Type: Adjust (increase / decrease), Direction: Decrease, Amount Type: Percentage (%), Value: 15.
5. Click Preview, check the sample, then Apply Price Update.

Exclude also works with By Category, By Brand and By Tag, for example to change everything except one category.