Honestly it doesn't seem that interesting - browsing /r/DarkNetMarkets, dnstats and just signing up yourself is enough. I've signed up for a few for shits and giggles but never bought anything, browsing is pretty interesting. You just need to find one person like me who'd be willing to give them the email... not exactly hard.

Perhaps it's from someone they were investigating for other crimes. I mean, it doesn't seem beyond the realm of possibility that law enforcement were going after say, a drug dealer who was using the site to sell goods, and found the welcome email from the platform he or she was using in their inbox.

That seems like one plausible possibility.

This is my pet theory. A seller was busted and the emails were siezed. Seems like the most likely possibility.

Isn't it likely that a number of recipients did not delete that particular message? Also that a number of them sold drugs by mail order, not uncommonly a short lived profession?

I'm sure they sign up for every darknet site constantly.