Bitcoin was only invented in 2008. Darknet has a much longer history than that. Given that Bitcoin first became a dominant currency on the darknet, and still is, what were darknet users using for a currency to buy and sell anonymously before Bitcoin was introduced? (Privacy coin Monero wasn't invented until 2014)
Using credit cards or PayPal accounts (stolen or not) weren't that private or anonymous and could be traced, so the majority of darknet shoppers didn't use them while purchasing products even before Bitcoin. There were other cryptocurrencies before Bitcoin that could have been used in darknet shopping. For example, as https://www.investopedia.com/tech/were-there-cryptocurrencies-bitcoin/ says, cryptocurrencies including B-Money, Bit Gold, Hashcash were there before Bitcoin. So there definitely were cryptocurrencies before Bitcoin and some of those could have been used though this might not be the case if the cryptocurrency in question is not so anonymous. Along with this, darknet shoppers could have traded items. They could have traded stolen credit cards, drugs, firearms, etc. with others. While darknet markets were there at the time, they didn't have as much of a name or attention until Silk Road came which made Bitcoin explode.
TL;DR: There were cryptocurrencies before Bitcoin that perhaps could have been used (like a beta version of how Bitcoin is used today) and products could have been traded. Cryptocurrencies weren't used as much before Bitcoin since the darknet markets weren't as large as they are today.