I think the issue is that the Seller wasn't 100% honest or upfront on exactly what the buyer would be purchasing.
The ad states
Have adapted a transferable conversion device to a SCAR (that is true)
Did it without modifying the host receiver (that is also probably true)
That the conversion device is not limited to a particular host or host family (ehhh...probably not so much based on the research of MGTony and the previous denial Historic got for these same triggers in a Bren)
There is an approval letter for this (I have seen nothing stating that a registered MAC Trigger can go into a SCAR, only that it can go into "any compatible semi-automatic firearm" and
compatible probably means any Open Bolt MAC variant...not a SCAR or a MK48.)
The conversion device can be adapted to any semi-automatic host with "a little" gunsmithing (I bet there are a lot of new custom FCG parts in a SCAR to make a MAC trigger work)
Like it or not, it is fairly well known in the community that the ATF has taken a historical dim view of using conversion devices outside of their intended family. I wish we could use sears in devices outside their family as I would happily buy a MGA/HK M249. I am sure that the XMG owners wished they could still use their uppers on full auto as well.
To me it feels like the folks selling the SCAR and MK48 conversions with these triggers got a semi-ambiguous letter back from the ATF on items they full well knew were designed as open bolt MAC conversion triggers and are trying to make a $100K capitalization on the word "compatible" and never asked for rock solid clarification by submitting a mechanical sample like Historic did before trying to find the fastest sucker they could.
Does this rise to the Ernie Wrenn RPD/249 upper debacle...probably not (and it certainly in no way compares to Clarke/Rodman/Greenburg fraud) but the folks who cooked up the SCAR ad MK48 certainly are not fully on the up and up in my estimation either.