Detailed Explanation
Background
A trustee, legally known as a bailee, is liable for the loss or damage of chattel or livestock a bailor entrusts under his or her jurisdiction. The bailor/bailee relationship is called a bailment [Exodus 22:6-14].
(The degree of liability is contingent on the degree of benefit the bailee receives from his/her supervision.
A. A gratuitous bailee receives no benefit for the service he/she provides to the bailor: hence is liable for no more than damages or loss due to bailee's negligence
B. A baillee for hire receives payment: hence is liable for theft or loss
C. a renter pays for usage rights: hence is liable for theft or loss
D. A borrower receives usage rights at no cost: hence is liable even for accidental occurrences) [Ibid. See Issue # 24 for Bailee Liability Chart].
☞ Take Off: Pre-Tarmac implicates the following law.
Generally, a gratuitous bailee and a bailee for hire assume responsibility and consequential liability upon meeting three sequential requirements [Choshen Mishpat 291: 2,3
1. The bailor requests the bailee to cognitively supervise the deposit,
1.1 "Can you please watch my article?" is an example of a request for cognitive supervision.
1.2 "Can you please keep an eye out for my belongings?" is an example of request for a mere associative level of supervision.
2. The bailee consents to cognitively supervise the deposit
2.1 Conversely, a request/consent for mere associative supervision lacks the basic ingredient for affecting a bailment.
2.2 Note: Circumstances often indicate whether a cognitive or an assosciative supervision was expected/accepted irrespective of the language employed.
3. The deposit legally transfers into the bailee's jurisdiction
[Means of legal transfer include
3.1 Bailee lifting the deposit
3.2 moving the deposit into the bailee's property or
3.3 in a semi-populated area; positioning the deposit within the bailee's exclusive four cubit radius.]
Application
Stern was preoccupied with his personal business. Green simply told him to keep an eye out for his belongings. At best, Green requested an associative supervision from Stern.
Simillarly, Stern exchanged a friendly smile. A friendly smile does not indicate consenting to cognitively supervise Green's belongings.
Green's ASUS remained three yards away from Stern. While the waiting area may be considered a semi-populated area, three yards is not within Stern's exculsive four cubit radius. Thus, Green's ASUS never transferred in to his jurisdiction.
Although, Mark Green offered Stern to pay him with a Cappuccino for his services, he failed to meet most of the basic requirements to affect a "bailment". Consequently, Stern never assumed responsibility to cognitively supervise Green's belongings and remains absolved from the mishap due to his negligent lack of supervision.
[Answered by The Fellow-Yesharim Research Center]