Tiny Tools › URL parameter glossary › mkcid
mkcideBay's marketing channel ID — records which channel a click came through (e.g. an affiliate network, email, or a partner site). One of the trio (mkevt, mkcid, mkrid) you'll see stacked on eBay links. Tracking metadata; safe to remove.
Safe to remove? Yes. mkcid is used for tracking and reporting only — deleting it does not change the page you land on. Strip it before sharing a link to keep it clean and private.