Yes this is a nice feature. I hope soon we can buy from those listings too.. for those situations where someone has listed a few items at npc / 1 gil, and you want to grab them and add them to your own listing

Another retainer management tip is picking "Sell Items from Retainer", then opening your inventory. This way you can sell items from your own inventory, retainers inventory and transfer items between the two, all without switching back to a different menu option.