I agree that this type of behavior should be against the rules, but spamming (yes, you were spamming) a person's trade about it is unnecessary. I'd advise reporting it and waiting for an admin to deal with it.

Could argue that what I did wasn't spam, as it was about the trade and not random garbage. Less one could consider ANY comment on a trade the OP does not like is spam

you can't possibly hope to regulate this; how can you be certain if its true or not?

If someone offers on the site, link to the trade or it is on the trade in question.
If someone offers in chat/on a server, a screencap would be fine.

Otherwise one can assume they are making it up

Or someone can ask a friend to post a fake offer directly on Outpost. You can't prevent this, if the current offer is suspiciously too high then they probably won't sell the item anyway since nobody will pay that much.

I believe this is a problem but even I were an admin; due to regulations of logic, there is no reason as to why it isn't entirely plausible to have an offer from the outside. Admitting that such astouding offers are not normal; we can assume that the trade will kill itself attracting to attention and thus the scammer is renderd useless.

Well, only reason that I saw that this was obviously fake is that He said he got an offer that EVERYONE who is selling BMOCs wants, but wanted more. Kind of clearly shouts out fake... But idk

Basically what norby said is easy to do and will be used as an alternative the moment a fake C/O rule is placed into effect. Plus, really, who gives a fuck, if you think the C/O is fake then don't worry about it.