In the CS is it possible to make an NPC sell a key? If so, what box should be checked in the AI?

Yes it is possible. You need to set the AI to offer services. Basically set them up as a merchant.

Ok thanks but whats the Buys/Sells box I have to check? Miscellaneous, Weapons etc.?

You can't buy and sell keys. You could buy a title to a property (as a "book" - miscellaneous item) and get a key with it, but the key itself is different. I believe it would get too complicated if you could ever sell a key, as the game doesn't have any way to undo the ownership that goes with it. So the game doesn't let keys be be sold or bought at all.

Ok thanks, how would you go about getting a key when you buy a certain book?

Actually I don't think keys can actually be sold --instead you have to set up a receipt like is done on the home sales in the vanilla game and then place a script on the book that when added to the players inventory changes the value of the receipt to 0 (so it can not be sold) and then adds the key to the players inventory. For my One Stop armors mod I first made the keys and then made a book\scroll for the merchant to sell (so the merchant was set up to sell books) -- then added the script below to the book -- then when the player bought the book from the merchant it would run the script (Only once so they did not get multiple keys) which changed the value of the book to 0 - and added the key to the players inventory. (NOTE : the same thing could be done in the background by making a new item (say food item or other item the merchant sells) and then adding the script to that item and adding the key to the player and removing the item that the script was placed on so that as far as the player can tell they just recieve the key. )

This is an example of the script I used in my one stop armors mod for the armor displays that the merchant sold :

Ok thanks JDFan....I think I understand that, it's funny because I can understand Java but not TES scripting.

^{Wait whats Doonce and Onadd?}

Doonce is a variable that is declared so it can be used in the script -- that way when the script is run the first time (Doonce is initialised as = 0) it passes the check in the line ( if ( Doonce == 0 ) ) so the rest of the script continues then once it runs the variable is set to = 1 so afterwards if the script runs again the check fails and the script does not run (without it the script would get stuck in an endless loop and run continually setting the recipt value to 0, adding the key, and displaying the messagebox over and over and over etc. !!)

SO it pretty much does what it says in the naming (makes the code run only once !)

OnAdd -- is a defined Block Type that runs when the item the script is on is added to the containers (in this case the players inventory) ( http://cs.elderscrolls.com/index.php/Begin )

Oh right, ok thanks just setting up the NPC to sell the book now

Thanks very much for the help

EDIT: Later that day.....Wahoo it works

Oh and I've set the price of the book to be 1000 gold but....it ends up being 1618 when I go to buy it. I know this is to do with the batering and everything but can I change it?

THe actual sell price in game is always going to vary since it uses a formula that includes several variables to determine the price including :

So I do not think there is a way to determine an exact price but you could adjust it so that the actual price that shows up in game is closer to what you wanted !

Alright thanks