Simply make it so free trial accounts cannot trade materials to other people once the marketplace is in. Allow incoming trades so if someone wants to hand the guy some gear etc they can. All materials, rares, gear etc are stuck on that trial character/bank. Make it so they cannot drop on the ground, give them a delete option instead until they are off trial. They can farm and buy everything from others when testing stuff out and only active subs can trade things they have gathered to other players OR sell anything on the market.