For auctions, I always snipe and use the free service offered by gixen.com for this.
For example, let's assume you see something you want to buy and think £40 is a reasonible price. Let's assume someone else thinks £30 is a good price, has bid £30 and becomes the high bidder at £1. If you bid £5, they become the high bidder at £5.50. You bid £10, they become the high bidder at £10.50, etc. until you bid over £30. Then you become the high bidder at £31.00. However the other bidder now might think it's worth more so he bids again and it goes up and up until one of you stop, say £40.
Now assume instead you have a last few seconds snipe bid at £40. You beat his £30 but they don't have the time to submit another bid so you get it for £31.00.
That's why I always snipe.
Regarding private listings, the only reason for a seller setting their listings as private is that they have something to hide so I will never bid on items from such a seller.
Michael