If you did not use the Zero Trace Pen for previous transactions you made a mistake. You’re probably not going to be traced back for this, but rather if you get in trouble later they can still find proof for your past orders. So, if I’m describing you correctly, then go ahead and wipe the computers you used and make entirely new market accounts.
The first step is to uninstall all the tools you used to order on your insecure OS. That includes the Tor browser, PGP tools, Bitcoin wallets, etc.
After that you have to overwrite the free disk space on your hard drive. You can use the free version of CCleaner to do this.
Note: This is not 100% secure. There are always log files that your OS might have created which still show what tools you used. Therefore it is important that you follow the steps mentioned above and keep everything related to the darknet on the Zero Trace Pen in the future.