I don't personally like the idea of bringing some random decoy over to 'test' my dogs for a potential puppy buyer. Sure, I wouldn't care if I knew who the decoy was and was confident of their experience and their safety, but some unknown PP trainer? Unh-uh.
If said person wants to come watch my dogs work with our normal decoy, has an idea of what scenarios they would like to see performed, and I think those scenarios are safe and wouldn't be damaging to either the dog's health or their training up to that point, I'd be all for it. I'm not trying to hide my dogs, but their safety and training are going to be my main priorities, and if someone has a problem with that, they probably aren't someone I want buying one of my puppies anyway. IMHO