Like Prop, I've bid at far too many auctions for clients to remember but one thing is clear to me. EMOTIONS CHANGE THE GAME PLAY
No matter how calm, cool and collected you think you are as a buyer, wait til your heart starts racing, your mouth runs dry and the sweat of anxiety breaks out on your brow when you are in charge of potentially securing your long-awaited dream home (or investment property- buyers still get emotional despite opinions to the contrary) in front of a huge captive crowd. Being harrassed/approached repeatedly by a professional selling agent during the process, encouraged/coerced by an award-winning auctioneer and stared down by your competition throughout is often enough for buyers to slip up and sometimes make the following errors (note: I have witnessed all of the below many times over):
1. Bid against themselves
2. Go past their limit despite best intentions not to do so
3. Indicate via body language or verbal means (mainly desperate whispering between partners, furious head-shaking, nervous nodding and numbered finger displays) when they're reaching their limit or about to pass it.
4. Watch the property sell before they've had a chance to call out their (often revised) "last" bid.
5. Re-enter the bidding after declaring they were "out"
6. Have the property passed in to them, and genuinely look terrified
7. Get on the phone during the bidding to ask mum/dad/sibling/friend for more money- a sure telltale sign that the limit has not only been passed but is about to be annihilated
8. Bid in higher increments than is necessary for the situation. A cool, calm and collected BA can sometimes save thousands in one ill-conceived and unneccessarily too-high bid by an auction virgin.
Regardless, and as others have said, of course you don't need a BA to bid for you if you are perfectly capable, confident and experienced. Negotiating after however is a whole new ballgame and when you're (often) facing 3-4 agents following auction across the table, it can be like lambs to the slaughter for those buyers who are unused to this type of negotiating.