I don't know about amazon-simpledb, but in database terms, you have two tables: the first is a table of USERS which presumably stores at least a user id and username; the second is a table of ITEMS which stores an item id, an item name and its current owner/user.
You do NOT want to store a list of items that the user owns in a delimited list in the users table; traversing this list will be painful. Apart from that, the data (which users owns which items) already exists in the items table.
In standard SQL, you would write the following query to retrieve a list of the items that a specific user owns
select items.name
from items inner join users
on items.user = users.id
where users.username = 'Luis'
Standard databases are built to handle tables with millions of entries so don't worry about efficiency (at least, as long as you add the appropriate indexes - you would need an index on the 'user' field in the items table).