Be able to make offers in auction house and shard exchange lower than the initial price.
Have a checkbox allow bidding offers.
If someone else has made a higher bid they are rejected.
The seller can decide to accept the offer at the end of auction or perhaps anytime.
If an offer is outbid or rejected it is refunded.
It could be a problem if the seller doesn't check their mail after the auction expires. I guess refunded after 3 weeks.
Someone might be selling something too expensive, buyer might not have enough coin. Someone else might be selling the same item cheaper.