While the Khajit are banned from the major cities of Skyrim, I was wondering how can I walk into the stores with Skooma at hand and not get arrested for it...well the main reason is because most of the storeowners in some way would gladly buy it off you
Whiterun: "Everything is for sale, hell if I got a sister I would sell her as well!"
Solitude: Demand by Argonian workers at the docks plus Gulum Ei
Windhelm: One is part of a rival Thieves Guild, the other sold a stolen ring
Markarth: If you are part of a cannibalistic cult, buying Skooma probably would be the least of your problems
Falkreath: The Jarl is a corrupt man, and what better way to show his wealth through buying illegal substances and possibly snorting it off of the bar wench?
Riften: City of crime, plenty of addicts to sell it to.
Winterhold: Given the city is in the butt end of nowhere and there is a couple of thieves guild associates and a guy who deals in black market goods
Dawnstar: The only minor hold with a Khajiit Carvan sitting outside.
Morthal: Quiet place in town.
I may post the reasons why Oblivion allowed you to sell Skooma at later date.