There are certainly subreddits to talk about various drugs, and subreddits that the darknet markets and how to use them, but as far as I know, there aren't subreddits for people to directly buy/sell/trade in drugs. Can you cite a subreddit where is a popular destination for drug trade?

Ditto, no drug related subreddit I know of doesn't explicitly ban dealing.

I won't out any, but while the rules are there, many deals take place using very thinly veiled codewords. Craigslist, mocospace, facebook, letgo, etc all have pretty blatant drug selling in the open as well.

Oh definitely: A friend showed me people openly selling weed on Instagram - no PGP or anything like that.