Okay, if you're in the Mage's Guild, do all of Ajira's quests. Nothing too complicated. Next, go to Suran. Go to "Desele's House of Earthly Delights". Its more of a nude-dancing hangout. Kill Desele, the woman behind the counter. Leave the Khajit alone. Once she's dead, loot the place for all its Skooma, Sujamma, etc.

If you did all of Ajira's quests, she'll like you a lot. She is one of the few people who'll buy Skooma. The Creeper is another, but if you're at a high enough position in the Mage's Guild, Ajira gives you even better prices (with haggling).