Mercury in 7th House is in the house of his friend Venus. Mercury does well in this house provided it is not afflicted. Seventh house is the house of marriage. First spouse is indicated via seventh house. Mercury is a known chatter box and sometimes also prone to use witty and critical speech. That does not bode well for marriage. But on the bright side placement of Mercury in seventh house is great for other matters concerning the seventh house.

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   According to B. V. Raman: "Natives with Mercury in seventh house are virtuous and friendly in approach. They dress well and have a taste for finer things in life. They have profound knowledge of law and quite skilled in business and trade tactics. There could be early marriage on cards and quite possibly to a rich partner. Natives are learned in mathematics, astrology and astronomy. They are religious and relaxed in approach. They may have a good physique and looks. If afflicted then Mercury could make the native cunning and deceitful". Seventh house is the house of first marriage. Marriage is a not just about romance and love. It also involves conflict management and healthy debates. Mercury excels in these kind of atmosphere though Mercury is clueless about affections and emotions. Mercury can compose great rhymes and poetries but underlying warmth and feelings are missing. But that does not mean it is a bad thing. If you carefully weigh the pros and cons then Mercury in seventh house is good for marriage also. Though it must be stressed that marriage is more of a business or contract and genuine feelings should be less. 

     If you leave the marriage portfolio aside Mercury placed in the seventh house is indeed a gift of Gods. Mercury is business minded, articulate, smart, talkative, quick thinker and learner and has excellent adaptive skills. Seventh house is the house of litigations, contracts, partnerships and other business related matters. Mercury is the best planet to handle these delicate and critical matters. Mercury in seventh house could make native a great success in the business sphere. Native becomes rich and successful through his own efforts and skills.

   Mercury with Sun in seventh house gives the Budh-Aditya Yoga and gives great results in business as well as government jobs. This assoication may give a beautiful and ambitious spouse. Mercury with Moon in seventh house maybe good for business but there could be misunderstandings in marriage. Mercury with Mars not only gives legal troubles but there could be rocky relationships in first marriage and a possible divorce on cards if native does not avoid conflicts with spouse. Mercury with Jupiter is a great placement and can give a succesful profession as a lawyer, lecturer or as a diplomat. Though there could be chances of putting on weight due to this association. Mercury with Venus is great for romance but not so great for career. Mercury's intelligence and smartness are wasted in pleasure and romance seeking qualities of Venus. Mercury+Venus in seventh house could also give a flirt, one who cheats or worse someone addicted to alcohol, drugs or other substances. Mercury with Saturn is not bad provided it is in a friendly house. Though it does give late marriage and there is a chance that native marries someone much younger or older then him or her. There is also a possibility that native is attracted to same sex as both Mercury and Saturn are considered eunuch and have trouble with gender identity. Kindly note it is not always the case. Mercury with Rahu and Ketu is bad for marriage and it could also give financial and legal troubles. There could be frequent fights with spouse. 

    Mercury in seventh house is a very good placement provided it is not afflicted. But in case it is afflicted there could be diseases of indigestion, skin problems, legal troubles and fights in marriage. Native should regularly consume food rich in Vitamin C and Vitamin A especially mangoes, papaya, spinach and other green vegetables to avoid skin problems. Indigestion can be cured by regular yoga and exercise. Also native can consume yogurt and other foods which promote digestion. Natives should pray to Lord Ganesha or Lord Hanuman if he is going through legal troubles. In marriage native should try to leave ego aside and try to sort issues to avoid problems later. Also native should be less critical of spouse to avoid conflicts. 

References:
Raman, Bangalore Venkata. How to Judge a Horoscope. Delhi: Motilal Banarsidass, 2014. Print.