"Post quality" has nothing to do with those discussions, they're perfectly valid for the board they're under. General discussion is meant for topics of that variety, it can be anything unrelated to AC as long as posts are relevant to the topic at hand and users are well-conducted in their discussions. A post quality infraction would more likely pertain to an irrelevant comment, derailing a thread and generally posting something that is not constructive to the discussion (such as a video or image response with no context).
As for prices in November/December, they were higher for the sets you're inquiring about because they were brand new and on high demand at the time. Sanrio items hadn't circulated the community very long back then, and being so difficult to get a hold of, many people wanted them. Now that people have been importing cards and obviously hackers have been selling sets, the demand for these items is far lower now than it was back then, so naturally prices will decrease as providers will try to undercut each other to attract more sales.
If you're looking for ways to sell these items at a consistent rate then you may have better luck selling RV visits rather than the item sets, though this is only an option if you have the cards. I haven't checked the Re-Tail board in a while but it was rare to see free RV visits at the time, that may have changed now. I've never really found selling items to be profitable on TBT in general, unless you take advantage of new items that aren't available yet. An example would be what happened with the white festive tree before Christmas; no one knew how to obtain it during the holiday season before it was announced for JP/NA, but hackers were able to exploit the high demand for them and sell them at upwards of 100 TBT. It was ridiculous at the time, and many users who took advantage of that have actually disappeared, I'm guessing because it was very clear that they had gained the item through "illegal" means on this forum. Now we're so far into the patch that it's unlikely that another opportunity like this will arise, most items have been discovered and are of equal rarity to pre-WA items.