What does the suffix mean in painter and seller?

Answer 1

a suffix used in forming nouns designating persons from the object of their occupation or labor : a "painter" is one who paints, a "seller" is one who sells.

Answer 2

In the words "painter" and "seller," the suffix "-er" indicates that the person is performing the action or function associated with the root word. In this case, a "painter" is someone who paints, and a "seller" is someone who sells.