Is it possible to accidently buy something from someone you have blacklisted?

Correct. You can. 

I think there used to be a warning, but there is so many ways to interact on Trade Me now I don't know if that's on all platforms or not. On Desktop, without clicking through and actually purchasing something I can do the initial bid, add to cart, or buynow without getting any warning I have blacklisted them.

Yes you can, You do no longer get the warning that you have blacklisted that member,

I have done it on my personal account, then same nonsense from seller then realized It is someone I had already blacklisted 🤦‍♀️, and trade me staff asked why did I buy from them again?, it was accidental because I had no warning they were a member I had already blacklisted quite a while ago, it was not until after I had used the buy now I connected it, too late by then,

They then started a new account as shocking feedback on other Intrade account, and dang it, I used buy now on their casual new account then realized same member, same payment instructions, that account was short lived when I alerted trade me,

They started another account, but I picked up on that as used same table again in some of their auctions with green paint splashed on it Lol!, that went poof quickly too,

and their Intrade account has now been disabled too, so am cautious now, and check location etc. if there seems to be similarities on auction listings, Once burnt twice shy!