If you happen to be building and someone walks by and decides they want to buy whatever you're making, I don't think anyone is going to come and ban your ass. Big difference between that and setting up an enormous, ugly store that monopolizes the sandbox and whose proprietor demands you build at least 5m away from the store's ugly walls (which I have seen happen before).
I'm not talking about putting up a "huge, ugly store", but nor am I talking about innocently having people offer for stuff while it's being built. I'm talking about leaving items that aren't being worked on unattended in the sandbox so that people notice them and notice the creator's profile, or leaving them on sale. This doesn't cause much disruption, so why shouldn't it be allowed?
Should we get rid of speeding laws because if I'm not going to be directly observed by a police officer, I'm going to go 10 MPH faster than legal and get to work sooner? Hey, I derived an advantage from violating that law, it must be useless! Bullshit. The law lets an officer put a stop to exceptional and obvious cases that threaten the public good, like idiots who zoom their Mustangs at 60 MPH in a residential zone. The no sales in the sandbox rule exists for the same purpose.
Well, if the real policy is "you can sell and display in the sandboxes as long as it doesn't disrupt other people or take up too much room" then why not put that in the TOS instead of the absolute ban that's there now? Then you woudn't get people who follow the letter of the law losing out.
Also, the example doesn't match on two issues:
a) arriving a few minutes earlier at work isn't that much of an advantage, and
b) if going 10MPH over the limit is safe, it's safe for any number of people.
In the case of a) being able to sandbox sell is a massive advantage. In the case of b), once a certain number of people sell in the sandbox it becomes full and this is then a major threat to public good.
How would you like it if every employer in your country announced that anyone who sped on the way to work would get a $100 bonus when they got there, and when you hopped into your car and accelerated, the cops pulled you over and told you that they'd calculated there was only enough space on the roads for 40 people to "safely" speed and you were number 41 so they were busting your ass?