What you describe would be off topic. You should report those posts and they should be deleted.
Yes, but that only applies for that specific scenario. A small change could avoid that:
The scammer could ask who can sell gift cards, and then his alt could offer their services, posting an auto-buy link.
Other alts could accuse the OP of trying to scam (to justify the warning at the top of the thread) and vouch for the first alt.
All of this while the thread is locked and nobody else can intervene.
The posts advertising, and vouching for the competitor (or "competitor" if they are alts) would be off topic, and should be deleted if reported.
I do agree there should be limits as to how frequently a thread can be unlocked. Perhaps one solution would be that once a thread has been locked twice in one month that if someone other than the OP makes a post in the thread, it can only be locked 4 hours after it was last unlocked.
There is no warning, perhaps not to count very old positive feedbacks, or to count only last few feedbacks?
I think the current algorithm is to prevent people like you from causing this warning being displayed for non-scammer reasons.