Stockbroking is the work a stockbroker does — the business of buying and selling shares and bonds for other people, for a fee. Notice the shape of the word: it is the activity, not the person. A stockbroker is someone; stockbroking is what they spend their days doing. So you say 'He went into stockbroking' the same way you would say 'She went into teaching', and you can put it in front of another noun: a stockbroking firm, a stockbroking career, the stockbroking industry. This form is more common in British English; Americans more often say 'brokerage' for the business — a brokerage firm, a brokerage account. Both are understood. The word belongs to newspapers, job advertisements and business talk rather than to casual conversation; nobody chatting with friends says 'I do stockbroking' — they say 'I'm a broker' or 'I work in finance'.

證券經紀業務、股票經紀這一行（指這個行業或工作本身，不是指人）