I missed an offer and wanted to contact the potential buyer. On their profile page there is no contact option "as advertised", at least not on a cell phone. And to search their ID, there is no "advanced search" option on a cell phone.
Try the mobile site instead of the app, if that's what you're using now. You should be able to search for a user by just going to http://www.ebay.com/usr/[username] too, and then contact them from there.