This is sent by the client to sell items to a vendor.

Offset Type     Name             Description
------ -------- ---------------- ------------------------------
 0000  BYTE     cmd
 0001  USHORT   size             The size of the packet
 0003  UINT     vendor           The serial of the vendor
 0007  USHORT   numitems         The number of items being sold
For each item, a structure containing
 0000  UINT     serial           The item's serial
 0004  USHORT   amount           The amount of the item being sold