using list as planning action parameter

Hello,

I am new to PDDL (and to planning / service composition in general), and 
I would like to ask the following question. Is it possible to use a list 
as a parameter for an action, when specifying the problem domain in 
PDDL? For example, let's say that I want to specify an action "buy", 
that allows me to buy a variable number of items (i.e., so that multiple 
items can be bought with a single action). Is it possible to express 
something like this?

Thank you for any help.

Dimitris