blockchain monitor??

[Markus11]

When i have for example 1000 Balls for sale, each 2$.

I put my public key on the hp so the costumer can send money for the amount of balls he wants.

So, how do i know if ball no. 1000 was sold and that i should put the articel status to "sold out?

Is there a api i can use or do i have to watch the blockchain 24/7??