Launching a Two Sided Marketplace

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Tips/Ideas for creating and maintaining a two sided marketplace?

I've already setup a working site and will begin marketing soon.

For the sake of simplification just imagine the marketplace is for people trading physical and digital Magic the Gathering Cards.

The site has an onsite virtual currency, so you can sell your items for it or buy other users items.

I'm thinking a small transaction fee on each sale would help pull the currency out of the site.

Also would allow users to 'cash out' to Paypal if they have x positive trades and at least $10 or $25 worth of virtual currency.

To jumpstart the site I'm seeding it with all my items and going to maybe give users something like $1-3 worth of currency. So not enough to just 'cash out' but enough hopefully to get a small item to test the site. Targeting sites about this niche where they already trade these but without a safe system or virtual currency.

Anti fraud measure so users dont just sign up a ton of times trading with their self. Going to require maybe phone verification like gmail does and the track users ips/cookies to try to prevent them faking trades to move around the starter free currency.

Anyone ran anything like this before? Have ideas why this will work? Why it will fail?

Many thanks