This is my personal opinion, but...
Do not bid on:
a) Private entries. Other than the very rare case where a private seller needs cash fast, selling at auction is a very bad idea for private sellers. So why do they do it? Well, they can't sell it any other way. I wonder why...
b) Nice, tidy looking low mileage part exchanges from general used car dealers. Erm, whats going on here? If this car was as lovely as it looks why is he banging it through auction at trade price and not selling it on his pitch?
c) Nice, tidy looking low mileage part exchanges from franchised dealers with approved used car schemes that are the same make. So..... nice little part ex BMW 318 here with 30k miles, 3 years old, direct from the main BMW dealer? What, so nice that he's not selling it via the approved used scheme? Whats wrong with it then?
Consider bidding on:
a) Finance company reposessions. Normally reasonably straight cars - but be careful, ****** who cannot pay their car finance don't normally look after cars either.
Great things to bid on:
a) Lease company de-fleets. You know the sort - ex company cars. 3 years old, near 6 figure mileage, all with full service history. Well looked after, properly serviced, going through auction becuase big leasing companies don't want to faff around selling them any other way. Great buys.
b) Main dealer part exchanges. Only when its stuff you can see why they are not selling themselves. So that means a lovely Ford Mondeo from the BMW main dealer. Of course it's not going on their forecourt, it's a Mondeo, they sell nice BMW's. Probably a nice car. Also higher mileage or older stuff - nice tidy Audi A4, but it's on a T plate and it's done 120k. It's not going to be sold by the main dealer, it's going to auction.
c) ex-motability. Tend to only be at big car auctions and probably out of your budget but if you just want a bog standard boring runabout thats nearly new, its hard to do any better. Under 3 years old, usually very low milage, full service history and still under warranty. Tops.