At market close, I will generate a random* number U from a uniform distribution between 0 and 1. The market resolves YES if U is less than the final market price, and NO otherwise.

For example, if the market closes at 70%, I generate a random* number. If that number is 0.65 (less than 0.70), the market resolves YES. If it's 0.85 (greater than 0.70), it resolves NO.
* See resolution criteria for "random" number

Resolution: The "random number" will be deterministically based on the number of traders in the market. I will generate it with the following code:
import random

random.seed(num_traders)

x = random.random()

print(x)