Some article ID numbers aren't actually trackable on a lot of sites (even Australia Post's registered service, both domestic and international, is not sold as a trackable service, even though sometimes you will get tracking results on some sites). 

In saying that, some "tracking" numbers aren't issued by any of the postal services at all, I also have a couple of items where the sellers have input a number, but they definitely are not tracking numbers or article IDs, they're internal barcodes / numbers generated by the seller, which serve various functions. They'd be better off uploading them as private notes to the order as they don't show on the buyer's side and can't give the wrong impression, but some sellers will put it as a tracking number because, while it won't help when official tracking is required, eBay's system won't automatically determine that it's not a postage service tracking number and it helps with some of the seller stats, like DSR ratings. (I'm not saying it's right, just explaining why some of them might do it - personally, I'd rather avoid annoying buyers in that way than try to get an automated rating).