@Jeffro - on Devot-ee the user account is created on the Devot-ee site and not the vendors site - though some vendors run their own sales sites as well. Vendors get their own support forum on Devot-ee or can provide a link to their own external support site. Devot-ee also creates addon licence numbers where they are required.

For the end user (developers purchasing free/commercial addons) it's a one stop shop for virtually every EE addon available, you can create different lists of addons to enable quick/repeat purchasing and download all purchased items in an order in a .zip archive if you want.