i wonder what is necessary for a trader selling and buying skooma except for being khajiit? is there a skript for that?
thanks in advance

It's in the dialogue (service refusals). To make a trader that will buy/sell skooma or moon sugar, you can either make them a different class than the ones the service refusals are filtered for, or you can add a blank line to service refusals filtered for your NPC (this blocks all service refusals below it, so you'd probably want to add some ID-filtered refusals above that for other conditions e.g. very low disposition).

ah okay. could you give me an example how to do this? i never did any dialogues - so i don't know exactly what to do but can get into it.

I may be off base on this but here's my understanding: In order to have service allowed requires that none of the service denial dialogs test true for the NPC in question. The only way to allow an non-khajjit to barter skooma the original service denial dialog would have to be eliminited. No mater what dialog you added above it would have to test negative for the NPC and when the engine got to the original service denial dialog that would test positive for any NPC that is not a khajjit. Thus, without removing the original test the NPC would use the original denial.

No, like I said: you can use an empty line to block all lines below (I've used it myself to block vampire lair service refusals with companions - it works).

@ Sister:
If you want to do it by avoiding the class filters you don't even have to add any dialogue. If you want to make, e.g. a "Trader" class NPC that will buy drugs, go to the Character menu->class and hit new. Create a class with a unique ID and for "Name" call it "Trader". Copy the other settings from the real trader class (or change them if you want). Assign that class to your NPC in the drop-down list and they should get the low disposition refusal as normal but won't get the drug refusals.

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To do it by adding dialogue (which doesn't involve changing the NPC, so you can do this pretty safely with original NPCs if you want):
In the dialogue window go to the Persuasion tab, "Service Refusal" topic. Slightly down from the top - e.g. on Blatta Hateria's refusal - right-click and select "new", then click anywhere else. You'll get a line that says "EMPTY". Don't add any text, just filter the line for your NPC's ID (Speaker Condition box, ID drop-down list).

The lines on either side of your new line will be marked with an asterisk. Change the text of those lines to something really obvious (I use "=== CLEAN ME ===") so you can easily clean those out of the mod later.

To make your NPC refuse service if they hate you, right-click on the empty line and select "new". For the new (top) empty line, enter some text (the default is "You're not the type I like to deal with.") and in the little box marked "Disp" (top right) put in 20 (that's the default) - this will make them refuse service if their disp is below 20.

Now save, clean the altered original lines and you're done. You can always add other refusals above the empty (e.g. for faction and whatnot) but anything below the empty will be blocked for that NPC.