Shopkeepers are immune to peaceful displacement as well

src/hack.c

@@ -1848,10 +1848,11 @@ domove_core()
                    && (!goodpos(u.ux0, u.uy0, mtmp, 0)
                        || t_at(u.ux0, u.uy0) != NULL
                        || mtmp->ispriest
+                       || mtmp->isshk
                        || mtmp->data == &mons[PM_ORACLE]
                        || mtmp->m_id == g.quest_status.leader_m_id)) {
             /* displacing peaceful into unsafe or trapped space, or trying to
-             * displace quest leader, Oracle, or priest */
+             * displace quest leader, Oracle, shopkeeper, or priest */
             You("stop.  %s doesn't want to swap places.",
                 upstart(y_monnam(mtmp)));
             didnt_move = TRUE;