A couple of people have posted the idea of a bazaar message for retainers or even one's self and while I support that idea wholely, I thought about another idea that could be used in addition to or in place should S-E not want to implement one for whatever reason.
How about an option to have a little message display telling anyone browsing your retainer your main character's name and, if you're a crafter, any appropriate crafting levels you wish to advertise? This way, say your retainer is selling... thingamabobs (very technical name, I know.) Anyway, Let's say MynameisBob Notsephiroth, we will call him Bob for short, wants to buy some thingamabobs and my retainer is out of them and no one in the market ward happens to be selling them. Maybe if Bob has bought from my retainer before, or perhaps if Bob is desperate for thingamabobs (oddly Firefox is not giving me a spelling error for this lol!), he could see that my retainer is advertising my crafting levels and he can send me a /tell in game asking me if I have time to make some thingamabobs or letting me know my retainer is out of them.
If I was not in game at the time, perhaps my retainer could take a message for me and the next time I check the retainer (would really like to have some way of "communicating" with the retainer without running to them physically) that message could be passed on to me. I could then make some thingamabobs and CoD them (whenever we are blessed with a mail system) to Bob, or I could arrange to meet with Bob some time to get him some thingamabobs.