If you're a traditional retailer building a marketplace, here's is one thing you need to wrap your head around: your seller is your partner.  Your best partner.

You attract more buyers, you take no inventory risk.  You negotiate no prices.  You take a cut, and give the rest to the seller.  You're in the software business.

You don't have to guess about what inventory to buy.  You don't have to guess if your merchandisers/buyers and inventory planners are good at negotiating or setting prices.

The sellers do all that.  Based on what the market will bear.  If they overprice their goods, yes you suffer too, but the seller suffers more. 

Outside of spending time understanding your buyers, spending time with your sellers is one of the best uses for your time.  Not only because your sellers create your experience.  But because they are more motivated than you are to make the proposition work.

And in particular I want to bring this point home to marketplace-builders.  Partnering with your sellers is not natural.  Most times, you are taught you need to "win" against your vendors.  A seller is not a vendor.

They are your partner.