If someone bought 2 items worth 2.1b each and comes out to a total of 4.2b sold. Would you receive all of it since the cap is at 2.1b.
The server checks if you still can possess anymore mesos everytime someone attempts to buy something. So no, after you sold a 2.1b item, if someone attempts to buy another 2.1b item, he won't be able to.
Basically this. People used to put up really expensive stuff (Fully scrolled cravens for example) for sale at 1 meso in the store and have the store filled to max meso. I remember it triggered me when I was little playing MapleSea lul.