The thing is , with internet searching , as opposed to the local paper copy of Exchamnge and mart, your 'shopping zone' is country wide , if not wider.
So if a good example of the model you set your heart on comes up 500 miles away , a couple of grand less than the others you looked at , watcha gonna do?
I guess there a max value for where "buy unseen" is a viable strategy - possibly the credit card fraud refund ceiling (is it £15k) or much less if that doesent apply.