One thing I would like to see to the existing system at least as far as guild trading is bids. Keep the existing buyout price, but add a bid system. When you sell an item, you could set an item buyout at 100,000 gold, but set a minimum bid. This might allow some more competition. Either pay the buyout price instantly, or over the course of the posted sale, you have people post auction bids as long as they have the gold funds to post the bid, and once a bid is made, to prevent any fraud, freeze any pending funds till the end of the auction. If you don't win the auction, any gold is returned to you. If you win, the gold you posted is transfered to seller and you get the item, and auctioneer takes the cut.
Example.
I put up a Torture Wheel for 175,000 gold buy out, but a minimum bid of say 125,000. 125K is the reserve price. If someone really wants it, they can just buy out the item for 175K, or you have an auction battle. 125K, ok I have a bid for 127K, "auctioneer rapid speak".... 135, do I hear 135?, 147 to the argonian in the red hat... 150, 150K for this sweet torture wheel, 151 from that orc in the corner, do I hear 155, what about 153, 160 from the dark elf, 160, do I hear 160... Anyone want to bid higher? Molag Bal made this torture wheel himself. It was made in hell, but is heaven! Going once, going twice, 170 from the high elf! Going once, going twice, SOLD! 170K!