Note that web advertisements generally assume broadband rather than dialup connections. So they are usually designed with pretty pictures or large animations, to improve their impact. As a result they can be large and take a long time to load on a slow line.
Alternative energy users - including both the actual buyers of the products you sell and the experienced users who provide helpful postings and answers to questions - tend to live in remote locations where only dialup is available. So adding ads is likely to affect them adversely - just as the large pictures did so recently.
When making a business decision on this you need to weigh the increased revenue from the ads against the potential loss of business from users who never get "hooked" by the BBS - due to their own difficulty in browsing it, or due to the loss of help from experienced RE users who might bail out. The former is easy to quantify, the latter much more difficult.
(Even when evaluating just revenue from the ads you need to consider whether your user base is sufficiently download-time sensitive that your community may collapse - or migrate elsewhere - if page viewing becomes too painful.)
