I understood that auctioneer's signs are for the benefit of the auctioneer and not the seller - when selling my own house in the UK I managed to negotiate a lower fee in order to permit them to put their sign outside my house. The argument is that serious buyers (where are they ?) look in the paper, the auctioneers office or the internet and don;t just drive around looking for signs.
I guess over here there is more of a need for the signs due to the lack of house numbers, road names, postcodes, direction signs etc