This is the structure for the default.xip file. With this you should be able edit the items NPCs are selling.
010 Editor Template attached below.
Code:
typedef struct
{
int category; // item type
int uniqueID;
} XIP_ITEMINFO;
typedef struct
{
char name[ 32 ];
int itemCount;
XIP_ITEMINFO itemInfo[ itemCount ];
} XIP_PACKAGE;
struct
{
int packageCount;
XIP_PACKAGE packageArray[ packageCount ];
} theFile;