There are a number of online communities for amateur astronomers. These forums can provide advice, camaraderie, knowledge and more. Many BAS members are also members of these online communities.
Cloudy Nights - a free online forum sponsored by Astronomics.com. Cloudy nights has thousands of active members posting on almost every topic in astronomy as well as a decent classifieds section for astronomy gear.
Astromart - a low cost ($12 one-time fee) online forum frequented by thousands of astronomers with a significant classifieds section as well as online auctions of astronomy equipment.