Sitting solid and capable amid her produce, ""Market Lady"" schemes: she'll capture the mangoes on the tree above her before they fall to the ground -- where by custom they would be fair prey for the eager children flocking around her stall. Installing her nets on other trees as well, she corners the mango market and sells enough to buy herself a Mercedes. Then, a nightmare concerning greedy hippos that devour her mangoes and threaten her own safety results in a Scrooge-like change of heart: ""You can't earn a living by selling what's free,"" she concludes. Pete Watson, who bases his protagonist on ""Mama Benz,"" a merchant he encountered while serving in the Peace Corps in Benin, holds attention with his brisk pacing and amusing details; his wife's boldly rendered paintings are full of vigor and good humor. For both, a strong debut.