I am looking for a word (preferably and adjective) to describe someone who only does something for others if he gets something in return, like a 'merchant spirited' person. I would like to use a word with a rather negative connotation. It is for a title of a book but I am not sure if the word 'merchant' carries a negative connotation or there is a better one.
The words 'merchant', 'tradesman' and 'businessman' are neutral (they have no negative connotations), but 'tradesman' and 'businessman' may be considered sexist in societies where female equality is valued, since they end in '-man'. A common negative adjective used about a person who does things only for something in return (especially money) is 'mercenary'.
interested only in the amount of money that you can get from a situation:
He had some mercenary scheme to marry a wealthy widow.
'Acquisitive' is an adjective that conveys the characteristic of being only interested in one's own personal benefit or profit. It conveys a disinterest even in the thing acquired. The focus of the word is on the mere act of acquiring, in and of itself.
strongly desirous of acquiring and possessing
Having the ability, propensity, or desire to acquire things; esp. excessively concerned with material gain; greedy, avaricious.
Volex in acquisitive mood