I know that when you have to beat team rocket in the underground path you end up in the basement of the mart and can get an item. I also noticed that there are several other rooms with items that i cannot get into... I was wondering how you get into them?

When in the basement, take the elavator too different parts of floors and come back to the basment. As time passes by, so does changing. The pokemon and workers downstairs change and work on fixing the place so must things that are on your way get move and can access items. Just do this till you can reach an item.