No group buy works like this.
My current dislike of vendor group buys is potential for bias on the vendor's side: Essentially when a GB occurs, the vendor chooses a couple people who want to sacrifice a vial, and then the VENDOR sends those vials in to get tested
The key above is that the participants never actually received their kits yet, so there is NO guarantee that the vendor didn't just make a juiced up couple of vials, pick a few of THOSE vials, and send them in, then send the ACTUAL kits with vials from another batch.
Chain of custody is everything, and if the vendor can pick/choose which vials to send to the testing company, no customer can actually be sure that is what they received.