There's a zillion things you can estimate this way. A lot of sites use sequential cookies, user IDs, etc. Until about a decade ago UPS tracking numbers were sequential for each shipper which made it trivial to estimate output for online shops. Apple invoice numbers used to be dense and sequential and you only needed the number to retrieve the invoice. The IMEI is actually just about the worst way to have estimated iPhone sales in 2008; at that time you could literally have crawled Apple's website for every invoice whether sold online or in stores.