Linden Lab has announced that migration from the existing Second Life Marketplace to the new Viewer Managed Marketplace (VMM) will start July 23, 2015. See: Viewer-Man
Migrations will start at 9PM PT and run to 9AM the following day. I expect this to take days, but may be not. You will get an email when they start and finish. Merchants with a large number (5,000+) of products will get advance warning of the pending migration.
After migration you will have to use a VMM capable viewer to add new products. You will be able to manage products via the Marketplace web site regardless of which viewer you are using. Sales of any given item will continue except during the time it takes to actually move the item to the new system, presumably a couple of minutes.
From the time your migration starts until it ends, you will not be able to manage products.
It is interesting the VMM viewer is still an RC version. This roll out date also seems to ignore the information Third Party Developers were being given as to when VMM roll out might happen. It also suggests that the VMM viewer will roll to the main viewer by the 23th.