A few ideas I had about the system:

1) Adding more wards to fight stability. Either Battlecraft 1, Battlecraft 2, etc. and/or maybe Linkshell owned wards? A gil sink where LSs can rent out wards or downright purchase them. (By renting that ward, it allows members of that LS to put up their retainers in stalls free of charge. But I would imagine it would be pricey to rent.)

2) Adding more stalls, a lot more, and making it required to be in one. This limits how many people can be in a ward as well as helps to get gil out of the game. The developers can work out what is a stable limit to each ward and make only that many stalls available in each.

3) Slightly off topic, allow players to see what others are trying to purchase from their retainers. That way we can see more than what people are selling, but also what they are seeking.

4) How about instead of just having the marking of the retainer option, maybe there is a choice to mark, or for the player to zone to the retainer. (Or possibly a stand where the retainer comes out to meet you.) Marking is good for people who want to search for several of an item. Insta-retainer would be good for people that just want to get in and out.