This is perhaps the most obvious category of crafted epics at risk in 4.2. Players will be able to purchase T11 chest and legs for 2200 Justice points on Day 1 of the new patch, and I find it very likely many will do so immediately with whatever amount of JP they have stockpiled. Do not forget that you will (still) be able to turn Honor points into Justice points, easily bypassing the 4,000 cap if you choose. Three weeks of ZA/ZG runs later, they will be able to further purchase T12 (same slots) with their Valor points.
The situation with belts is more complex. Players will not be able to purchase any belts with JP nor VP this patch. Instead, there are two other sources of belts: a small sample from the Molten Front dailies after ~25 days, and a complete set after hitting Honored with the Avengers of Hyjal.
The three belts in the Bad category are Elementium Girdle of Pain, Light Elementium Belt, and Lightning Lash. All three have ilevel 365 analogs available after unlocking The Armorer in the Firelands dailies. The rest of the belts will not be replaced until you get a raid drop or hit Honored with the raid reputation. Last time I checked, the word on the street was that you could hit 11,999/12,000 Honored with the Avengers from farming trash - how difficult this will be or even if it is possible will remain to be seen. I find it likely however, that someone farming Firelands raid trash will probably want to do it with a purple belt in the meantime.
Already talked about this before, but...
The Darkmoon trinkets have given me a lot of AH mileage so far this expansion, but I stopped making the cards a few months ago. Players logging in after 4.2 will have the option of getting 359 trinkets immediately for 1650 JP and/or doing ~25 days of dailies for the 365 trinkets off of the vendors there. And if they hit Revered with the raid reputation, hey, more trinkets.
That covers the crafted BoE epics, but why stop here?
There are no crafted epic cloaks, but I am referring to any BoE epic cloaks from T11 content and/or ZA/ZG drops you may have in your bags. Sell them. Sell now.
Although I missed the chance to warn you before the Midsummer holiday arrived with its free 353 cloak giveaway, the 353 and 359 cloaks were still technically as good or better than what you could get
If 365 cloaks from a quest chain was not bad enough, you also have Valor cloaks for JP and 378 cloaks after however long it takes to get Friendly with the Avengers of Hyjal. Move those capes like they're hot.
Specifically the BoE epic boots you can buy with Valor today, and Justice on Tuesday. Seems pretty obvious that these would be bad deals to get, yes? Well... not quite. Ask yourself if you would be willing to buy Justice points for gold at, say, a 1:2 ratio. In other words, imagine spending 8000g to get 4000 JP. Would you? I would. You cannot get 4000 of course, as the boots are the only items that work this way and they cost 1650 JP. Point being, even though I can grind up the 1650 JP and get them "for free," I could also simply buy them off someone who values their time less than I do and spend those same JP getting something else. Similar to belts though, there is a small kink in the plan: boots from dailies.
The three boots in the Bad category are Boots of the Perilous Seas, Rock Furrow Boots, and Moccasins of Verdurous Glooms. All three have 365 upgrades available after a scant 25 days worth of dailies. The other boots will not be replaced outside of raid drops so you will probably be able to sell them on a decent basis. Of course, so will everyone else capable of running heroics/grinding BGs but if you have nothing else to buy with those currencies, it is better than buying and vendoring Wrath epic gems.
Is anyone still making this garbage? Please, stop, what are you doing?
The mat price for the 346 rings/necklaces was ridiculous even on Day 1, but there were still chances at sales and (low) profit margins. Generally people will tell you to make the Elementium Moebius Band because it's the cheapest but actually pretty good for any tank. Problem is that there is not one, but two epic tanking rings from the Firelands dailies. From the same vendor! I guess Blizzard's thought process was that since the one ring had Parry on it, that a 2nd one was necessary for the Feral tanks. Except now if you are a Plate tank you have easy-street access to both. Did I mention they are on the same vendor? Non-casters are in the same boat with two separate Agility rings AND two separate Strength rings... on different vendors this time, but still. Spellcasters and/or healers got the shaft though, with only one ring.
So you have the option of the Valor rings being bought with JP, the new Valor rings bought with actual Valor, the crafted blue PvP rings having a difference of a whole 90 stats for the equivalent of ~1500g in mats, the 365 rings from the Firelands dailies (which covers every spec), and eventually the 391 rings at Exalted with Avengers.
Necklaces are in a mostly similar place as rings. Spellcasters get a Firelands daily option as do Strength DPS however. Missing here are Agility and Tank necklaces, which means you may have a market for Brazen Elementium Medallion (assuming people didn't just get the Hyjal reputation neck) and Elementium Guardian. There are Valor necklaces available post-4.2, but I doubt the neck slot will be at the top of peoples' lists to replace for quite some time.
Conclusion and Caveats
The mark of a good goblin is being able to make gold when it is not especially obvious that it is possible. Although I am labeling some items as Bad and Fire Sale and so on, it is entirely possible that you or someone who likes to sweat by the AH will make money where money ain't got any right being made. Maybe that dude who bought my cheap cloaks will move both by Tuesday and laugh his way to the bank. That's fine, I'm already at the bank giggling myself. The only real loser is whoever gets stuck with the item when the music stops, and presumably that person paid for the privilege so I guess everyone wins... until that guy does those 10 quests and replaces it.
If you are looking for some quick and dirty advice, I would say: clear your stock, stop making/buying more BoEs. Worst case scenario you will still have your mats, and can go back to ignoring whatever I say on Wednesday. Goblins strike it rich all the time by taking on risk that no one else does. Goblins like myself also strike ~80% of the richness of the other goblins, with 90% less effort and less than a quarter of the risk. Do what you like, and we'll all see how it pans out next month.