"Big ticket" items, like the Insight books and WT-CD, are not kept "in-stock". You have to special order them.
(1) It's not very likely the dude behind the counter would take your order if you are not a member of the congregation, and (2) even if he did you'd have to wait a month for the stuff to be shipped.
For things like that e-bay or similar is your best bet.
If you want stuff that is kept "in stock", like Bibles, song books, books used in field serve-us, etc., you could probably get some stuff by posing as a "Bible study" or "unbelieving mate" or "visitor whose mom is 'in the truth' ", etc.