The Merchant Prince of Sina by Robert Van Dusen

Rumor has it that the wealthiest merchant in the exotic port city of Sina has an uncrackable vault in his manse. In fact, he's so confident of this that he's offering their weight in gold to anyone capable of opening it. So far all have failed to claim the prize...
Sarah Walker, a penniless thief and adventurer, spends her last ducat to get to Sina and try her hand at gaining a fortune.

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