I'd want to use a GUI as an interface for a shop like Final Fantasy ones
I'd want the character to be able to choose from a list and by clicking on one of the item buy it decreasing the variable "score"
How can I make a list like this on a GUI?
Characters can have a certain amount of items, which can be set and retrieved with character[CHARACTER].inv[ITEMNUMBER] - check the manual for more info.
Anyway, you could use an NPCs' inventory to achieve this. When your shop GUI is displayed, run a routine to check the displayed items and have labels display the item number. GetInvAt() would probably come in handy here.
If you don't have much AGS experience, I'd recommend making a simpler game first. RPGS aren't easy to do in AGS.