To loaf around or dawdle on the job; to waste time instead of working (literally, 'to sell oil').
仕事中に油を売ってばかりいると叱られるよ。
If you keep loafing on the job you'll get scolded.
彼は営業所で油を売っていると評判だ。
He's known for dawdling around at the sales office.
会議中に油を売るのはやめて、議題に集中しよう。
Stop dawdling during the meeting and focus on the agenda.