If the rules are to negotiate the trade on the post, why do so many people pm to talk about trades? Also, when I ask people to comment on the post, they rarely do.

If people refuse to post in topic, I would be careful and not give first/use mediator.
Trade topics are for "aproving trade".
Pms are for questions/discussing.

You are not allowed to Discuss in trade topics (not even as OP) "no back to back posts" - As this bypasses the 4 hour bump rule.

Most times, it’s them hoping you don’t know value, or wanting to snipe on the low side to flip rather than be immediately outbid. Hence why they don’t want to post in thread. Sometimes it really is to simply not wait/in a rush.

Personally I think knowingly attempting to “rip off” some via pm is akin to scamming. Many disagree with that opinion.

I ignore offers like that completely.

Now someone Pming you to say “hey I think your item is on the high side for bin, and wondering if you will take x instead of y” is just to not spam thread sometimes.

If they won’t post, it’s almost certainly my first statement.