Some genealogical speakers bring products to sell when they speak. There are a few challenges. They have to sort out local sales taxes and things like that. But it can prove a lucrative revenue stream especially if the products are your own. One speaker who does this is Dr. Penny Christiansen, a colleague at the National Institute for Genealogical Studies. Penny has written umpteen books on genealogy topics, published by Heritage Productions in Toronto.
Books are not the only products offered by speakers. I have seen CD-ROMS, DVDs, and even T-Shirts!