Anecdotally, Planet Money looked into this years ago and their reporting was that as far as they could tell, drugs on Silk Road weren't less safe than street drugs. Most of them were likely "fell off the truck" samples from the original manufacturers being sold by people with an in on the supply, but no otherwise-easy access to an out on the demand.
Their observation was that reputation mattered on SR a lot and a well-kept reputation was valuable at scale in a way that it isn't for being a street-corner pusher looking to stretch your buck by cutting your supply with adulterants. The smart play was to provide a high-quality product at a reasonable price (the latter being the easiest part since they were bypassing the obscene markup of official channels).