Specify how much, how and when youll be paid. The how much is usually an acceptable percentage of the gallerys sale price or a set price. If the gallery wants to grant a 10 percent discount to customers who purchase multiple works, work out whose part will cover this discount. For the how, the proceeds from a purchased piece should be held in trust until youre to be paid. Require a periodic accounting of what was sold when and whats still left. The terms for when youre to be paid should be set for a reasonable time (30 days after the purchase, on the 10th day of the month for the previous months sales, and so on). There should also be a penalty if the gallery fails to pay on time.