the biggest hurdle to overcome in such platform is setting up and establishing effective dispute system which would protect the business, sellers and buyers
associated risks would probably add-on ebay like commission rate.
Step 1 (the hardest in the bitcoin community):
Do reasonable steps to positively identify sellers and eliminate fake registrations. That could be with a copy of an ID and a small bitcoin payment. If you do not prevent people from making fake registrations and giving themselves positive feedback through multiple accounts you will have too much fraud.
Step 2:
Tracking # based escrow. Funds are released when a valid tracking # goes from seller zip to buyer zip. This does not stop all fraud, but it stops a good deal of it. A feedback system can provide a rounded protection for the buyer.
Step 3:
Fees. Please charge, but not too much. This stops spam/junk auctions. Make it be MUCH less then ebay but just enough to stop crap from being posted. Maybe 10 cents (in BTC) to list and 1% final price.
Step 4:
Advertise and promote a series of absolute auctions (shipped by auctions site itself) of items people want. There will be some money loss involved. Space them at one every other day for a month. This will generate lots of traffic.