Used to be able to do it - it's how I got my YZ250D - but I haven't figured out how to do it nowdays, unless:
1. They've listed their phone number/email in the ad;
2. They're selling something else (and you contact them through that ad);
3. You've contacted them before the auction ends, and have their email address...
But remember, I'm no Ebay guru.
With the number of scammers out there, many sellers seem automatically wary of anything off Ebay. It used to be that adding your phone number helped heaps (makes you a real person/accountable), but that seems less effective in recent times.
Freaky, it was a 71 model.