Based on my limited knowledge of traditional astrology, the 7th house represents the other person, our partner, romantic or business and a host of other matters. So if there’s a Mercury in your natal 7th, it could suggest that Mercurial themes such as communication, student, transport is associated with your partner.
After numerous consultations with clients over time, it seems that the 7th representing the partner is not encompassing enough. I have found repeatedly that a person with say Mercury in the 7th could mean that the person thinks or talks about relationships all the time or with a therapist. Open enemies a.k.a. people who openly object to him/her could do so with words, contracts, speed, etc. If the 7th is taken to represent our partner instead of say open enemies, the 7th could represent anything about our partner – our relationship needs, how we experience our partner, the physical appearance and personality of our partner, etc.