Each transaction is assigned a unique transaction Id/order number by the payment processor and also a unique invoice number by us. Both of these are visible in E-junkie transaction logs.
Since PayPal transaction ID that buyer and seller see for a single transaction is different, we need a common identifier and we use "invoice" for that. It can't be sequential as we don't want anybody guessing that unique identifier.
I would argue that e-junkie numbers are sequential based on a specific formula they have developed. So no jail time for e-junkie or I will defend them in the court of France - then I will go get more time in at the Louvre.