As of 22 January 2013, referral link posting is subject to a per-user throttling that is determined by the quality of past referral deals.
As of 14 June 2017, referral link posting is no longer allowed, (exception explained here). These referral posting limits are only applicable in rare circumstances or for users posting links or mentioning their third party/price comparison websites.
As of 24 July 2018, third-party posting limits are imposed per third-party site as well as per user. If multiple associated users are posting third-party links for the same site, the posting limit is shared between them.
As of 22 March 2021, third-party posting limits are combined with the third-party site's own store posting limits. Please refer to that wiki page for posting limit calculations