There's a couple of possibilities:
1. If the stall is open then they chose to say No when asked if they want to put their items into the stall network - just buy the sword directly from the stall.
2. If the stall isn't open (I've forgotten to click Open Stall occasionally

), then it won't be in the stall network and you won't be able to buy it until the player notices and opens his stall.
Hope I understood your question correctly and this answered it.