How do you find the best time to buy and sell a stock?

Question presented to candidate: "You are given a list of daily stock prices. You may buy one share on one day and sell it on a later day. Write a Python function that returns the maximum profit you can make, or 0 if no trade makes money."

What a strong answer should cover:

• The order constraint: the buy day must come before the sell day, so max(prices) - min(prices) is wrong.
• The one-pass idea: keep the cheapest price seen so far and the best profit so far; O(n) time, O(1) space.
• Why it is correct: for each sell day the best buy is the minimum of the earlier prices.
• Edge cases: empty list, one price, falling prices (answer 0).
• Python details: float('inf') as the starting minimum, and why min(prices[:i]) inside the loop silently makes it O(n²) (slicing copies).
• The unlimited-trades follow-up: sum of all positive day-to-day differences.

Clarifying questions expected:

• Exactly one transaction, or many?
• Should a falling market return 0 or the smallest loss?

Code / implementation expected: Yes — a single for loop over the list with two variables, in Python 3.