Personally, when buying, I prefer the auctions that do not have internet bidding as, in my opinion, its use has tended to push prices up as you get more bidders. Selfish? Of course it is but I prefer less competition.....
I have never sold via Holts but have bought from their Sealed Bid auction; got a couple of real bargains as well, even allowing for the commission on buying.
And beware: VAT is usually only payable on the buyers commission but, and a big but, if you buy a product that has come straight from a dealer and is classed as 'new' (i.e. had not been sold by the dealer and is not s/h) then you pay VAT on the hammer price, plus, of course the VAT on the commission. Auction houses should make you aware of this before bidding starts.
Cheers, Phil