Here's my take on announing sales.
While I would love to know exactly when something I want is going to be available, it's just a dream. Announcements aren't going to help most people much anyway because it's still first come, first served with Mr. ruling all.
I don't have a problem with somebody starting a thread with the initial post just stating their sale terms (payments, shipping, etc.) and giving a general description of what they'll be selling if they're going to start posting the books shortly (like within the hour). I also don't have a problem with the first post only containing a few books even if the seller intends to list 1000 more. There are no guidelines for how many books can be listed in a single post and different people use different formats.
Posting a thread, or responses in a thread, that contains nothing for sale (whether nothing has been posted for sale or everything has been sold) takes up space and helps bump other threads off the first page. The only exception to this is the Probation thread, which (sadly) is needed and has no other appropriate place.