It looks like Alphabay's forum is making you use the main site's same username now since they ask for a registration token and shit. That's new, and totally bullshit.
I'll make a new alphabay account completely and use that on the forums too then. I can already smell "We don't like you talking truth in the forums so we banned you from the forums, and since we link it to your main account now we banned that too!"
If you don't like your purchase history being linked to your forum posts, which is a very valid concern, you should simply make a second account that you can tie to your forum account and not order anything with.