I'd ask what the person means, because I don't think it is a really well defined term. I've seen some for sale posts that clearly indicate they are calling a male a "proven breeder" just because he's locked with a female. IMO, that proves nothing, because he could possibly be infertile due to some reason such as no live sperm.

Once you know what that particular person means by proven breeder, then I'd say it is a lot like buying hets. If you trust the person for reasons such as them having a good reputation, then you can trust them on this as well. If it is just some random person off craigslist or something similar, then I'm not sure you can even trust a pic of the female on eggs. There is still the possibility that none of them were fertile (rather small if the eggs look good), or the babies all had defects.