they are.
They will also take things in completely different points. Like information they didn't include on the topic they started as an insult to their intelligence. Theyre a rather nasty bunch, and get banned pretty quickly.
I've been working the marketplace since it started. (and the gaia exchange then too.) the amount of time that I've spent watching prices, has given me at least some intuition on item prices. (although it is far from perfect, and quite a few items have baffled me as to how they don't follow the rest of the trends despite all of my calculations, the Mochi and Momo being two such Items....and any evolving item....)
the big thing to look at, is what people want. One thing about holiday items, is that they're supposed to be more expensive before the event, however, people don't buy them if they weren't already going to get them. Everyone else waits until the actual event and uses those.
Store items, game Items, and box items, have a more stable price. (while box items have fluxtuated depending on ow many were in selling.)
event Items being a much more widely appearing item, have been steadily decreasing in starting price, and older items have been steadily increasing in price. there will be jumps, but no one who doesn't already want the item, will buy it then. Often the increased prices revert after the item does not sell. and will continue the slow increase.
quest Items are similar to holiday items and will increase very slowly due to the large quantities.
There is a sort of season, but it is best explained by calling it hopeful ambitions and not an actual season. (the inflated prices do to net more gold, the truth of the matter is that there is a smaller amount of the Item that gets sold.)