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.
That would be arguable at best, up to the moderator.
But it's not even too important as the OP could explicitly bump the thread as if he were asking for more providers.
Yes, someone creating a thread soliciting suggestions for services would get around the "off topic" problem.
My solution above regarding forcing threads to remain unlocked for a period of time would solve most of the problems regarding fake vouches.
You can also open a scam accusation, naming the forum handle, the URL of the service and/or contact information (eg telegram handle), so they will show up on google search results.