This is most likely a reason why SE is making new seasonal stuff unique/untradable/market prohibited
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Actually, they have been making season items sellable via the MB that were once Marketboard Prohibited.
Items include, but are not limited too, Starlight Sentinal trees, Paissa Doll's, Cherry Trees etc. That's why you'll get many players that buy "extra's", store them on their retainers and then sell them a few months down the line for eggstraordinary prices. When they add them to the MB as an item they get unlocked.
I'm not sure that they would do this for barding however. This is more something I would expect them to put onto the Mog Station. I think they had the old Christmas barding on there at one point?
I hope they open up the doll stand from the Little Ladies Day event eventually. I bought up a ton of them. :P Though I intended to get a couple as I didn't know if they were the kind of item that would get destroyed if you change houses or whatever.
It used to be that items that weren't untradable (ie., you could put them in a trade window) would be allowed on the MB after the first patch following the event putting them in the game. That policy changed with 3.2 (was it 3.1?), along with enabling event gear to be put in the armoire immediately.
What old seasonal furnishings? The "Market Prohibited" tag has only existed since 3.2, less than a month ago. We've only had one (completed) seasonal event since then, and its only furnishing was Untradeable as well as Market Prohibited.
(Actually, I was kind of wondering about that new "Market Prohibited" tag, as it generally seems redundant. I've yet to come across anything that's tradeable but market prohibited.)
It exist longer, but was invisible before 3.2
example:
Empty Twinkleboxes / Sheep Dolls / Hard Rice Cakes / Stuffed Qiqirn / Starlight Wreath / Star-topped Starlight Sentinel / Moon-topped Starlight Sentinel / Sun-topped Starlight Sentinel / Snowman / Empty Gift Boxes / Kadomatsu / Starlight Interior Wall / Flame of Passion
Those items were tradeable since they came ingame during their events (Starlight 2014 - Valentines Day 2015), but sellable in the market board only since 2.55 (end of mach 2015)