Stock in trade
The literal meaning originates, unsurprisingly, in the stock being held in readiness for sale. It now encompasses stock as in shares as well as services. We currently tend to apply it more often in reference to a person’s particular assets, a technique or ability habitually used by an individual or group. For example, a novelist might refer to an assassin’s special set of skills as his stock in trade.
Quote: Glamour is what I sell. It’s my stock in trade. Marlene Dietrich.