What is it?
A buying limit is a limit you can impose on how much of something a person can buy. When you see new products like the drink Prime that are really popular, a buying limit is imposed to stop resellers from profiteering.
Why does it work?
A buying limit works because we only place limits on things that are very popular (or expected to be) to make sure as many people as possible can benefit. When you have a buying limit, you imply an expected level of demand or demonstrate an actual level of demand which incites FOMO in your buyer.
How can you use it?
If you have a product – this is the most obvious usage. Identify a limit of 1/2/3 per customer and advertise it to promote genuine scarcity and imply expected popularity. The limitation has to be per person rather than the supply overall to make this work.
If you have a service – there is no obvious alternative, but you can highlight the number of people you work with/serve as the limit and focus on FOMO