I am going to answer this from the perspective of a gallery artist of almost 10 years... in my experience the public often finds the gallery environment intimidating, and are not sure of how to interact with gallery staff or how well they will be received.  Often galleries are perceived as cold, sterile environments that do not welcome casual browsers, but instead cater only to well-heeled patrons.  For the average person, galleries that are warm and inviting with down-to-earth curators and directors seem to generate more sales (with the exception of course of the rare and sought after multi-million dollar patron of the arts making vast purchases from the most well established galleries and renowned artists.) In addition, artists who are willing and available to interact with potential patrons often make a personal connection that leads to a purchase.  As an artist, making myself available to potential buyers at the gallery greatly increases the chance of a purchase.  In addition, I have found that once a visitor leaves the gallery, chances of a return for purchase are quite slim.  The secret seems to lie in creating an inviting atmosphere for visitors, engaging visitors without pretense, and artist availability to interact with potential buyers.  As an art buyer, I have indeed purchased art from a gallery on a first visit -  on one occasion, the gallery was out of state, and even though I was not familiar, the gallery was welcoming, and the artist I was interested in was on hand to talk about her work and process.  We walked away that evening with a piece of original work we cherish, and a personal connection with the artist that created it.  In contrast, I have left a gallery without purchasing work I found interesting after being treated poorly by the staff or artists, and have ignored galleries with a reputation of being unwelcoming to guests.