Interesting trend illustrated here in the guidelines for Ashley Grayson Literary, the agency where author-turned-agent Lois Winston (“I knew her when–and we even exchanged manuscripts for critique”) works. They’re not the only ones saying this.
From their Publishers Marketplace listing:
Note: New submissions policies
Published authors of fiction and non-fiction: We are still accepting clients who are previously published by a known publisher (no self-published or print-on-demand).
Unpublished authors – Fiction: The agency is temporarily closed to queries from writers who are not published at book length (self published or print-on-demand do not count). As of September 15, 2006, there are only three exceptions to this policy: (1) Unpublished authors who have received an offer from a reputable publisher, who need an agent before beginning contract negotiations; (2) Authors who are recommended by a published author, editor or agent who has read the work in question; (3) Authors whom we have met at conferences and from whom we have requested submissions.