How do the billboards know when a customer has purchased an album?

Answer 1

"SALES

Data for Billboard's sales charts -- which include all of our album charts -- are compiled by Nielsen SoundScan from a universe of merchants that represents more than 90% of the U.S. music retail market. The sample includes not only music stores and the music departments at electronics and department stores, but also direct-to-consumer transactions and Internet sales (both physical albums via Internet, and ones bought via digital downloads). A limited array of verifiable sales from concert venues is also tabulated.

All sales charts use the entire Nielsen SoundScan panel, with the exception of the R&B/Hip-Hop charts which uses a panel of core stores that specialize in the genre. The Nielsen SoundScan system utilizes that same point-of-sale that music merchants use to track their inventory, so you can think of itemized receipt from your last trip to the music store as a ballot cast for our charts."

this is as stated on the billboard site

Answer 2

The Billboard numbers aren't based on retail sales. They are the number of units shipped by the distributors (Sony, WEA, Universal....). That's why a CD can open at number one (or whatever) on its release date.

Answer 3

UPC universal product code, combined with inventory

Answer 4

Because there is a percentage paid to them for each purchase.