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...
Personally, I think all this NC750 stuff is overblown. We have some internet post of dubious quality/origin. While it may be true, it also may not be, and people can come up with lost of reasons for and against. In the end it doesn't really matter. What will happen will happen, and life will go...