Fix AH 7-item limit not being enforced correctly

[Kreidos]

AH listing count is now checked at every sale, and not just the cached value from the last time player viewed their Sale History.

As previously discussed, the limit is enforced on the client so there's no reason to make it a configurable value.

Fixes #2673

[Kreidos]

This would also alleviate a common cause of #3425, where players can list way too many items and overflow their mailboxes.