Honestly, the more I think about the the more I like the idea of keeping the current retainer system and adding a central purchasing location. Something like what im about to list below would keep people on both sides of the fence happy, I would think
Keep the current retainer system as it is, nothing will be changed for those who want to continue to manually browse it.
Add a central purchasing 'location', be it a NPC or anything else. Basically give everyone a main menu they can access and use to see all the available items being posted by all retainers on the realm. This would tie the three current main cities together.
Let people use that central location to buyout items as they normally would through individual retainers. From that point you could have the item sent to your inventory, or (and this is what I would prefer) they could add a mail system or mog house for the items to be delivered through.
This system would give everyone a fast and easy way to effectively browse through all the items posted up by the retainer system. Players would be able to sort through to look for the best prices and make their purchases from there.
The sales portion of the system would work the same as it does now. Players post up items through their retainers, and pick up gold from sales in the same place. The 'central location' would simply be tied to all retainers.
The people who like to browse could still browse, and the people who want an easier way to do it would have it.
How does that sound? Are there any negative sides that I am not seeing? Is there *anything* bad about that idea that would harm the game or peoples gameplay at all?