I just recommended getting the BBB seal to a client after watching conversions rates increase by nearly 2% sitewide. Tracked conversions on that particular site for a year, and within 3 days of adding the seal noticed the increase in conversion rates. Average conversion rates were 3.32 percent before the addition of the seal, (from search engines) and 4.01 percent from PPC campaigns before the addition of the seal.
Conversion rates remain a consistent 5-6% throughout the site with the goal being 9%. (we wish) :)
The BBB listing is certainly one that yields authority and trust, although some folks feel they are at easy risk for a smear campaign with the BBB due to the ruthless retail markets they service. However, if all is calm, then I think its an invaluable -- and FREE -- tool to help bolster credibility.
Another interesting concept is a listing with your local Chamber of Commerce. This is nothing new and of similar credentials to the BBB, but I've found some Chamber of Commerce websites will list your URL. What a nice perk, and we can all use the extra PR for the proverbial icing on the cake of credibility.
The drawback is a listing with the Chamber of Commerce costs money (sometimes big bucks depending upon your area) whereas the BBB is free. Me thinks the CoC will welcome your membership & yearly fee, but at the rate they charge in some states, it'll be difficult to pump more commerce back into them. :)
The BBB online program is not free unfortunately. The fee for the reliability seal will vary depending on the local BBB branch, but count on a one time fee of close to $200.00 USD. The privacy seal fee is based on your annual sales, and ranges from $200.00 USD per annum for sites with less than 1 million in yearly sales.