Wordpress is written in PHP. Its originally an "article management system" where you can work on a blog and publish it on certain dates, with comment. Much like a lot of newspapers work.
Anyway with the certain plugins such as woocomerce you can make it to a shop. But because wordpress is so frequently used its usually a target by many hackers. But otherwise I would say it _should_ be safe.
The only bad thing is that some people have javascript turned off so wordpress might not work for them. Because yes js is a thread for vendor I dont think the gov would play its card out (using a tor browser js engine vulnerability) for a random vendor but I guess they would use such things rather to catch pedophiles or terrorists because if their trick becomes public it will be fixed by the community (open source tor).
I would say Wordpress is fine even for a group of vendors sharing it except if you plan to create a new white house market then you should start worrying about not using js, have multiple hidden onion services (to support more users at once), captcha to throw bots away...
Hell a lot of people dont even realize how much security WHM had. For example they even checked if your request came from a real browser implementation, it took me a long time to realize how they made it:
What I did was resending all request to their server and responding it back. You can use this to represent a complete original site but the only factor you change is the btc deposit address... Its basically a man in the middle. No I never used this to phish anyone, reasearch only. First I used a simple JAVA server which was only redirecting the query to the original server. This didnt work and I had no clue why. When I tried the same with nginx it worked fine. Then I found out how they did it... They checked if the headers started upper case as per definition. Most implementations had lowercase header names. But the tor browser and most browser used "upper case" names eg. "content-type" vs "Content-Type". So I used python raw socket to and build my own http fucking reuqests! And it worked like a charm.
For example Dread has no such protection so you could easy write a phishing site for Dread.
As you see there is much more shit going behind the scenes regarding security but even if you sell about 20'000$/day I wouldnt worry much. So just go nuts and use wordpress but let a professional set it up for you.