[Closed] Buying a car from an Auction (BCA etc not ebay). tactics and pitfalls please.

The family car is due for replacement pretty soon and being based in Calderdale, I have the large BCA Brighouse site close by. Their website lists the different sales that are put through with fleet companies selling off their stock and even manufacturers shifting stuff (recent BMW group sale). I drive a company car and while it is serviced when needed and pretty much money no object maintenance given, fleet etc tends to be classed as being thrashed.
Just wondering if anyone has secured a good deal and what precautions they took to not buy a dog. I know that aftermarket warranties are available so would the "cheap' purchase plus the warranty still be a saving on a forecourt price?
I'm thinking of getting along when relevant sales come up and figuring out where sales prices end up against used car price guides.