Help please :) Unique ID as a product, for an online outdoor game. What is the most simple solution?

I am creating a running course (a running race without specific start time and date), with startpoint, checkpoints and finish line. I need to register, control and award the participants. My idea is to sell them the ID as the product. So costumer buy the ID, what can be use as a secret code to check in with QR codes at the subpages, as he/she follows my track. I need to register the time at startpoint, at checkpoints and at the finish line. So I will have a database with the total time, and I will have a winner.
Question 1: How can I sell them a specific ID code (a number for example)?
Question 2: How can I collect the timestamps I need (at the checkpoints I will set up QR codes, which directs them to a subpage of my site, where they have to write their ID number)?
Question 3: How will be these datas visible for a ranking for every participant?

Please, if you have any idea let me know :slight_smile: