Purchasing USJ Express Pass at Ticket Booth

I referred to USJ website and it says their express passes are sold out on the days i will be visiting. By any chance do they reserve extra passes to sell at the main entrance?

Likely not.

"Tickets can be purchased on the day at the park, but are available only while supplies last. Tickets will not be available on the day if they are sold out beforehand (excluding some tickets).
http://www.usj.co.jp/e/ticket/express_pass.html

"パークでの当日販売もありますが、売切れ次第、販売を終了します。また、事前販売で売切れの場合は、当日販売はありません。"
http://www.usj.co.jp/ticket/expresspass/#ucj

The Japanese version does not refer to "(excluding some tickets)".