Although not the fastest war to "farm" silver coins, as such - could you just invite a friend who already has 50 silver coins into your world as a multi-player environment, and then the merchant will spawn assuming you have the right house for him?

I am making an assumption that a world made available on multi-player will still be available for you as a single player afterwards, (Edit: I know this is correct now, as I have tried it - as long as it is your world to save and play)

Edit: as Camster points out below, they would have to have 100 coins, and give 50 to you - as each player needs to have 50 coins before he will spawn.