Agree with all the above. A tip here to avoid bidding accidentally, and yes I made an embarrassing purchase once
don't look at the auctioneer in the eye and move around or nod whilst doing so, avoid eye contact if you don't want to bid (they have eyes like hawks). Make sure the feet are good and the mouth, look out for undershot or overshot ones. Also try and find the breeders before the sale, make sure to get there early so you can really have a good look around and a good chat to each breeder about their stock, this will also be a very good way of finding out which you want and which you don't want. Maybe the title should be a beginners guide to buying at an auction, not an idiots, don't be harsh on yourself
All the best and enjoy yourself
If you're not sure then take a friend who knows whats what with sheep. It is really what is appealing to your eye, what I try and do is envision what they would look like on your holding, try and picture them and see what happens. Very often when I have bought rams and the EBV's are the same or very similar, I look to see if they have clean bellies, another plus especially shearing wise, and picture what they would look like as a flock or in the rams case I picture him within the flock and if i can't see it I don't buy him, sounds weird I know but that's how I chose my ram last time. The EBV's where the same the only thing he had over the other ram was the clean belly and the fact I could picture him in the flock. Sometimes though performance can go over what you see as being in your flock, so for example if his genetics are better than anothers and yet you can't envision him in the flock go for him anyway! it really depends on circumstance
Soz for the lecture. Hope you have a nice time and do let us know how you get on