They put it on sale for less. The way the system works is this: The lowest sale price will sell first, always. The auction is basically against other sellers, rather than by bidders. So if you put it up for 10,000 and someone puts theirs up for 9,999 (or even 1) their one will sell before yours. This is why I generally subtract about 99 from the price I hope to sell at - and someone might bid 901 on a 1k item, but more often than not they'll bid 1k and I'll get it ahead of anyone who has their items on sale for over 901 gil.