I know people are going to hate me for this, but I might go with a fee style system too of 1% of asking price, non refundable.
So to put an item for 100pp up on auction, you'd need to have %1 (or 1 pp) in advance to pay the fee before it gets listed.
I'd be using a custom table to store everything, item price, item id, seller, etc.
I don't know if there would be 'bidding' yet.
Listing "Flowing Black Silk Sash" for sale by all vendors sorted by lowest price first is something that would be super easy to do in mysql.
Since I played GW2 recently, I'd probably use their Auction House for ideas and examples of how players can buy, sell, and search.
Would have to wait until after T7 though