Bad things galleries do to artists: Unethical galleries will take in a piece of artwork by an artist, and when the price is discussed, the gallery says: "What's the price?" and the artist says: "$1000" The gallery nods OK and the artist leaves, knowing that if sold, he'll get $500 (most galleries in the DC area charge 50% commission (in NYC some are as high as 70%). The gallery then sells the piece, but for $2,000, sends the artist a check for $500 and pockets the extra $1,000. That is why artists should insist on having a contract with a gallery, and the contract must specifically address that the artist will get 50% of the actual sale price.
Bad things artists to do galleries: A reputable gallery gives an artist a show, and goes through all the various expenses associated with doing so (rent, electricity, staff salaries, publicity, ads, post cards, opening reception catering, etc.) So far the gallery has put forth a considerable investment in presenting the artist's works. An interested novice collector meets the artist at the opening and expresses interest (to the artist) in buying some of his artwork. The artist, wishing to stiff the gallery for their commission says: "See me after the show and I'll sell it to you directly and save myself the gallery commission." This is not only unethical, but it's also guaranteed to ruin the artist's reputation in the city, as these things always come out in the wash, and soon no gallery will exhibit any work by this artist.