The DEA prides itself on big hauls and tends to leave small stuff to the locals. Success in their line of work is measured in kilos and tons.
I can see them making a bit of noise about SR crackdowns to discourage use of the site, but it's hard to imagine them dedicating a ton of technological resources to a relatively tiny number of people selling each other dime bags over the Internet.
I could be wrong, but it seems like it would be a waste of a lot of time, energy, and money for very little return. Granted, that statement could apply to all drug law enforcement.
That's exactly why they would do it, they need to justify their budget if they want to keep it. Going after "a webring of drug dealing kingpins" goes over well in the press and in their budget review.