As a published author I find having a website and having business cards at my talks help ease this. It's great if audience members buy the book, but consider these reasons they might not: public libraries are tax-supported and seen as providing services already paid for from our taxes. Audience members usually don't know whether or not an honorarium has been paid or how much it is. They come for service they've paid for. Another reason people might not buy right then (in addition to cash flow or uncertainty as to how they can pay - do you take credit/debit cards? Only cash?) is that they have an existing relationship with a bookseller or may prefer to get an e-book verson. Take your business cards. You may be pleasantly surprised.