If you want to avoid the small walk, you can save the expense of putting a summoning bell in your house, you can just put your retainers there instead. This is the practical advantage, from my point of view. Few people will go around the residential area shopping at retainers when they can go to a market board and see all the retainers.

By the way, an in-home market board, like the in-home summoning bell, would be a nice addition. The only drawback is you wouldn't meet that much your neighbours at the market board, or residential areas would maybe look emptier, but it would be very nice to have the possibility of having the market board inside the house.

If you want to buy from a friend (maybe more expensive than the best buy) you can use his retainer if he has a visible one, usually you don't know the names of your friend's retainers. Not the most probable use, though.

There's also the visual thing of seeing retainers, the gear they have, etc. as others have said.