A few people have been scammed on the site, Only use paypal to pay for items for sale by other members. If they will not use paypal, its likely a scam NEVER SEND E-TRANSFERS OF ANY KIND.
To be honest, I have never heard of a helmet squeaking like that before. I would say it would be a cause for concern if the squeaking is coming from poor workmanship on any seems or loose/poor attachments for the straps or padding. It could also be that inner linear may be loose/poorly fitted...
Welcome to the forums and enjoy your new bike. A possible cheap/easy fix for the seat is a pair of padded bicycle undershorts. For me, using these has doubled the time I can stay on the seat without extreme annoyance from about 25-30 minutes to nearly an hour. As I haven't been doing any riding...