Yesterday I had just unlocked Completionist Cape on an account, and went to the market to buy it. Before doing so I had forgotten to ensure there was enough space, so when I bought the cape, the 1 million gold was just lost. There was no warning indicating there wasn't enough space or anything to this effect.
Cleared out my inventory of crafting supplies and bought at again.
Today I went to double check, perhaps I overlooked something, so I went ahead and tried buying another cape with a full inventory, it went and stacked onto my existing cape. So I sold them both, filled the slot and bought it again, sure enough, no warning and the gold went into the void.
Opening up an inventory slot and re-logging did not bring back the cape, nor the gold either.