in the case of popular fiction, sort of like Apuleius, I understand that there were publishers who would mass produce manuscripts for a large paying audience. But what about pamphlets like the writings of Christian apologists and their pagan opponents or let’s say philosophical speculations of Marcus Aurelius? Was there a paying market for this to justify mass production of manuscripts? Did the writers actually expect to reach an audience beyond their cafe acquaintances?
Also, what do we know about advertising for books or pamphlets back then in the absence of regular newspapers or magazines?
Mass production had a very different meaning then. Basically you had a copy house with people copying your manuscript by hand. No printing press, and very expensive. So when people met, there was generally one copy. Advertising was by word of mouth - likely the voluble mouth of the town crier.
And a lot of works got distributed verbally - it’s similar to the situation described in Farenheit 451, where people are preserving books through memorization. There were people who would read books to groups, and people who woud memorize stories (either from a book or from another storyteller/sage/whatever) and recite them. In Spain, romances de ciego (“blind man’s poems”) were memorized by blind folk, and their recitation used to make a living, as recently as the mid-20th century - they could be romances, horror stories, shocking news… or, if someone had bothered set a political pamphlet as a sarcastic poem, political pamphlets.