"Please only repost your advertisement if it's been at least 15 minutes since your last advertisement OR there has been at least 15 messages in the trade channel since your last advertisement."
This is from BB post... there is very importand word here - "PLEASE". Beeing familiar with law acts in my job, I don't see any rule here - BB just ASKS politely to do not put your trades too often.
Check the other points in this post - there are "prohibited", "can't", "not allowed", etc words which makes rules, but PLEASE do not make any rule.
I'll ofc not spam trade with my post, but if any over-law-abiding kid gonna put warning like this " pls rememcer the new rule is same posts every 15 minits now on trade" every 10 minutes, I'll report it to BB as a spam.
And remember - common sense is the best rule maker.