So, lets see what Mercury can do in 7th house without any conjunction or aspect from any other planet. 1st lets see what these 2 things represent --
Mercury represents communication skills, marketing, calculative ability, younger siblings, hobbies, skills with hands, intelligence, quick decision making, logical thinking, youthfulness etc.
7th house is house of market place, other people (masses), business partnerships, agreements, marriage, spouse, marital happiness etc.
Career wise, it is a definite career in the field of Law because 7th house represents agreements, contracts and Mercury is a counsellor. So, quite clearly it is a person who is involved in Legal Counselling.
7th house is house of Marriage and Mercury is Communication, so it indicates that this person wants to have a very communicative relationship with spouse. Spouse would be very youthful and childlike.
Businessman can surely be seen from this position as 7th house is house of Market Place and Mercury represents communications and marketing itself.
Another career which can be seen with this position is of Marriage Counsellor or someone who us in work of match-making like Marriage related websites.
Hope this helps.
Tomorrow - Mercury in 8th house, another position where Mercury can surely take you in Astrology.
Vishal S Saxena - Astrologer