Because chemical analysis is a solved problem, peptide synthesis is commercially trivial, and black market sellers live and die by their reputation.

Additionally, grey-market supply is guaranteed to be real, because anyone can sell it for nonpatented use cases and there will be a huge demand by researchers for it, as biochemists don't often do their own peptide synthesis.

Turns out selling real things is good for business, that's why the black market is filled with real things.

It's also filled with a lot of garbage. How many people are going to be able to verify that the "modified FOXO4-p53 interfering peptide filled brownies" they got from Silk Road are genuine? The effect isn't going to be immediate, and the placebo effect alone might make it seem as if it works.

Probably the same way they verify their drugs potency. There are a lot of 3rd parties in these market places that provide services such as chemical analysis of vendors products and make these results available. In fact, it goes as far as the very vendors selling the drugs pay to have their goods independently verified and issued with a "certificate" of how good their quality is which they display on the product listing.

These black markets or silk roads are not just a bunch of drug swinging deviants. A lot of the processes are very well thought out. As someone already mentioned their business relies heavily on their reputation so they tend to take care of it quite closely.

> There are a lot of 3rd parties in these market places that provide services such as chemical analysis of vendors products and make these results available. In fact, it goes as far as the very vendors selling the drugs pay to have their goods independently verified

Who verifies the "independence" of the "3rd parties" and/or the reliability or honesty of their results?

> As someone already mentioned their business relies heavily on their reputation so they tend to take care of it quite closely.

Reputation is important to any business. Hence the practice of dishonest/insincere/bought and paid for reviews or endorsements.

Simple. You also buy a bunch of old mice.

With these sorts of things you have no idea how to tell a real from a fake. Which is why even the supplements on Walmart's shelves are often altered. Nobody knows if the drug they get is real or is a gelcap filled with flour.