The term smart is from late Old English smeart "painful, severe, stinging; causing a sharp pain," related to smeortan (see smart (v.)). Meaning "executed with force and vigor" is from c. 1300. Meaning "quick, active, clever" is attested from c. 1300, from the notion of "cutting" wit, words, etc., or else "keen in bargaining."
Being smart in sales means meeting creative, quick, and clever. Being smart in sales also means trust and transparency. People buy from people. People buy more from smart people.
Causing a sharp pain, unattested cleverness, bargaining power.