Is this for staff responses only?
When someone comes into a FS thread to vouch for the product or the seller, the first thing I do is look at post count and then the location of the poster. If they're both newer members and near each other... it's a useless post to me. It's almost worse than not even "vouching" at all.
Now, if you're an authorized dealer and the guy is selling something with a transferrable warranty, all the seller needs to do is put that info in his OP.
I know you said you're just trying to keep people from getting screwed over, but coming from someone who's been scammed 6-7 times, your addition to a personal friend's for sale add would give me zero peace of mind.
Getting infracted is annoying, but don't sweat it, bro.