Welcome to the forum!
You can gauge the sale by a few smaller metrics such as:
- How old is the members account that you are buying from
- How high the post count is, because generally the more they've put into the forum, the less chance there is of being scammed.
- Look at the member's feedback score to make sure there's no negative marks.
After that, since there's no sales thread, the best you can do is trust them & hope it works out. You can message a mod to be extra safe, but I'm pretty sure they don't do that kind of thing unless I'm mistaken.
Good luck. There's also this thread: https://www.blackhatworld.com/seo/avoid-being-scammed-basic-pointers-for-your-security.306301/