This system is one where the first person to the sale site gives himself/herself number one and waits for the next person to show up and passes the duty to them as number 2 in line and so on. This is a self policing type number system that is open to abuse. For instance number 2 could take numbers 2 through 7 and give the next person number 8. Then give the numbers 3 to 7 to friends at some point later and poor old number 8 would have actually been number 3.
You can see why this system is not one favored by most people. Some companies honor this system until they arrive on site, starting their number system where the self start numbers left off. You should know the policy of the company holding the sale.