Q: Someone gave me a piece of jewelry to sell for him. It was stolen. Am I liable for the theft?
A: A broker (sarsur) who is entrusted with an item to sell becomes a guardian. We know that a paid guardian is liable for theft, whereas an unpaid guardian is not. Therefore, the question is whether you received potential benefit from tending to the sale, even if not directly for guarding the item, and even though the gain was not actualized (C.M. 291:1; 303:2).
Thus, if you were promised a fee for the sale you are a shomer sachar (paid guardian) and liable for theft. Similarly, if the owner set a price for the sale and it was agreed that if you sell for more you can keep the extra, you are a shomer sachar, since you can earn profit. Even if the owner gave you a gift, which was clearly on account of the brokerage, you become a shomer sachar (C.M. 185:1, 7; Be’er Heitev 185:4).