They can do this by shilling. The shill account bids a very high amount, thus discovering the highest legit bid. This bid is then canceled by the seller, and a bid driving the price to the max legit bid is entered from a shill account. Foiling this is one of the good reasons for sniping.
The way I tried (and it has been many years so cannot remember exactly how I fugured it out) was to increase price by small steps…and it showed some bit of information when you within a certain price of the max bid. Cannot remember what it was…but I was able to do it.