HN counts to 3:

  1, 2, 3, 4, 4/1, 8/2...

:)

I figured it was common enough for employers to check since they can buy the data, but I have no idea.

They can kind-of sort-of somewhat buy some of the data but the data sources they can buy from have errors, don't account for stock compensation, often misaccount for money going into 401K's or stock purchase plans, healthcare and other payroll deductions. They sometimes mishandle bonuses as well. If the data you buy has these errors you might reject a candidate whose not lying because their total comp was half stock and they maximized their 401K and stock purchase plans making the bank underestimate their salary.

What data can they buy, and how do they buy it?

Equifax has a salary lookup service. https://krebsonsecurity.com/2017/11/equifax-reopens-salary-l...

They only have data for those employers who opt into it though. It's mostly entry-level-type positions (supermarkets, convenience stores, temp agencies, etc) but there are some larger clients in that pool too.