Hello,
As one of this Forum's Necromunda Rules Enforcers, I have referred your questions to my fellow enforcers, and you will be no doubt hearing back from us shortly with an official reply.
Now, taking off my official hat, I will say that the needle rifle is most likely a mistake. GW have always had a problem with summary sheets and tables, so I'd say it's a typo, but I will take this up with the other Enforcers, and we shall solve all your problems shortly.
On gang weapons, the trade table tends to fall secondary to normal rules about gangs. For instance, you could buy a basic weapon for a ordinary House Juve, but they couldn't use it until they are a fully fledged ganger, and they are able to choose from other weapon types!