Explanation

This is a yes/no interrogative sentence in direct speech.

Direct:

The salesman said to me, "Are you interested in the scheme?"

Rules for indirect speech:

"said to" becomes asked (since it's a question)

The question form is changed to a statement form

"Are" becomes was

Use if or whether for yes/no questions

So,

Indirect: The salesman asked me if I was interested in the scheme.