Q: I placed my shopping bags outside the supermarket and promised a boy $5 to watch them while I got my car. One bag was stolen. Is he responsible for the theft?
A: If the boy did not touch the bags, there is a dispute on this issue. The Gemara (B.M. 99a) teaches that just as a purchase requires a kinyan (act of acquisition) to confirm the transaction, a guardian also requires a kinyan to assume legal responsibility for the item.
The Rambam (Hil. Sechirus 2:8) accepts this ruling literally. Tosfos and the Rosh maintain, however, that once the owner relies on the guardian and leaves the area, the guardian becomes obligated, even without a kinyan.
The Shulchan Aruch is inconclusive on this matter (C.M.  291:5, 303:1, 307:2, 340:4), but the achronim lean towards the opinion that a kinyan is required (Shach 291:13; Aruch Hashulchan 291:14). Therefore, if the boy did not make an appropriate kinyan on the bags, he does not become legally responsible for them. This is true even for a shomer sachar, although he forfeits his payment if he did not watch properly (see 301:1). To assume legal responsibility, the boy would have to pick up or move the bags, have them placed in his property, do a kinyan sudar, or give a handshake. However, he does bear a moral responsibility to pay if the theft was caused by careless negligence (Aruch Hashulchan 291:15; Imrei Bina, Hil. Pesach #5).