In vanilla 1.02 you almost always get units from buying or swapping. If the town/castle can train some of your units from it's buildings, you'll probably get some of those. Otherwise if there are mercenaries in the area, you might get some of those (Good way to get elephants in the ME). Only if neither of those is true should you get no units.

In my AAR I've been swapping my away across Europe, and I've gotten troops every time. Even at Durazzo, where there were no buildings at all, I got merc Knights.