There’s a reason that legacy publishers reach out to social media rock stars with book deals: the rock stars already have established their own platforms. Even better from the publishers’ perspective: those who do routine speaking gigs to hundreds of people.
Its simple economics. Moving books en masse (even at a steep discount) means much, much more money than “onesie twosie” sales.
Unless the publisher is offering a massive advance, I can’t for the life of me understand why any speaker would use a legacy publisher. Why make $1.50 or so per book (even when sold hundreds at a time) when you can make three or four times that much? Also, there’s the control factor. Publishers (even and especially big ones) have been known to bungle deliveries for author book signings and speaking events. One author friend of mine who routinely had that issue with her publisher.
She wasn’t pleased.
If you can move books en masse, why bother with a legacy publisher?
Brass tacks: if you move tons of books at events, training sessions, and the like, negotiate a large advance (easily north of $10,000) or just say no. You’re holding all of the cards.