  Dark web marketplaces (DWMs) are online platforms that facilitate illicit
trade among millions of users generating billions of dollars in annual revenue.
Recently, two interview-based studies have suggested that DWMs may also promote
the emergence of direct user-to-user (U2U) trading relationships. Here, we
quantify the scale of, and thoroughly investigate, U2U trading around DWMs by
analysing 31 million Bitcoin transactions among users of 40 DWMs between June
2011 and Jan 2021. We find that half of the DWM users trade through U2U pairs
generating a total trading volume greater than DWMs themselves. We then show
that hundreds of thousands of DWM users form stable trading pairs that are
persistent over time. Users in stable pairs are typically the ones with the
largest trading volume on DWMs. Then, we show that new U2U pairs often form
while both users are active on the same DWM, suggesting the marketplace may
serve as a catalyst for new direct trading relationships. Finally, we reveal
that stable U2U pairs tend to survive DWM closures and that they were not
affected by COVID-19, indicating that their trading activity is resilient to
external shocks. Our work unveils sophisticated patterns of trade emerging in
the dark web and highlights the importance of investigating user behaviour
beyond the immediate buyer-seller network on a single marketplace.
