The difference is that in the cases you mention you know the price you are going to pay in advance. In this case you don't. The price changes as you spend more

The difference is that in the cases you mention you know the price you are going to pay in advance. In this case you don't. The price changes as you spend more money to bids. You have no way of knowing what the final price will be and you are gambling on whether you'll get a good price. I repeat, if you have infinite money you will get the item if you want but that's not what you are gambling on. In a real case, if there are two or more people as determined as you, it is impossible to predict who will get the item. All will be spending money in advance without knowing who of them will get it. That's why it's a gamble.