When I had to take a few months’ break from Fiverr, I messaged a seller to ask her if she would be okay with me referring her to some of my buyers. So I suggest you first confirm with the seller you’d like to recommend that they wouldn’t mind your recommendation. Of course, you can’t know for sure if they will have as good a relationship with your buyer as you had so there’s always a risk that things won’t work between them. If you’re concerned about the rules, I can tell you that I’ve had people recommend me to others before or mention those who recommended me in their messages, and to my knowledge, nothing bad ever happened. If you’d like to be sure, you can always message CS to check.