I am new to BA and trying to learn standard practices. Is it normal/accepted to look at the members who have selected a beer as a "Want" and send them a message letting them know I have that beer and looking to set up a trade? In this case, it is a relatively little known beer and only a handful of people have selected it as a want.
I wouldn't have a problem with something like that. If they don't have any interest in trading then no harm really.
This seems about a million times more advocate-y than the reverse which happens all the time: sending unsolicited, usually low-ball, offers to people who list a "got" of something you want.