我觉得不宜把这里的more than视为一个习语，似可这样分析：
he was nimble in the calling of selling houses for more than people could afford to pay＿.
在动名词短语selling houses for sth结构中介词for的宾语more似可解释为比较项，而比较分句than people could afford to pay中pay的宾语是其比较对应项，被强制省略，语意大体为“卖房子索要金钱，超出人们支付能力”，这里的calling，根据柯林斯词典，似可解释为“vocation(职业)”，介词词组of selling houses for more than people could afford to pay是calling的限定性后置修饰语。
个人理解，仅供参考。
