Reference the comment on CAP and Glasses. It is a bit of a two-way thing, there is no doubt a drop in the guides makes buyers more cautious over what they pay but, yes, ultimately the market will decide. The bigger issue though is that banks, finance companies, stocking loans etc. use these guides to value stock held by a dealer. If 10-15% gets taken off the values then they depreciate that sock value by the same amount and businesses who had a positive balance sheet on Friday have a negative one on Monday. That makes banks nervous about overdrafts, makes it more difficult to borrow and makes stocking companies very worried.
If you're a bank sitting on a stock loan book of £1bn, or a leasing company expecting 30,000 end of lease cars back in the next 6 months seeing 15% taken off the value of it all overnight in terms of book price is not good news. Of course what ultimately matters is what they actually realise in terms of cash but short term it makes the financials look suspect.