Core, are you saying that the rise in the housing market is closing the gap between the value of auctioned and non-auctioned properties and if so what kind of percentage figure can you put on the difference?
Yes - gap is closing as "inexperienced" (and possibly foolhardy) individuals join the race to buy properties to let. In the NW of England recently over 2,000 people are estimated to have tried to get into an auction for property that used to get an attendance of a few dozen - the frantic and unexplained lunatic prices (double the auction guide price) have impacted all properties in the area.
As a guide to the difference it is impossible to say as regional variations are significant.
For those buying property at auctions they should ensure they get a solicitors sellers pack for the property and stick to a price as highest bid they calculate beforehand and not get carried away by the euphoria of the auction.