TL;DR your server's economy and AH ecosystem are going to be shockingly different after the realm mergers. This can be both good and bad depending if you are a buyer or seller. Be prepared to make changes to your selling style.
So it has been a week since my server Silvermoon-Mok’Nathal was merged and wanted to shared what has happened since. Prior to the merger prices for most everything were fairly constant day-to-day. Once in a wile someone would drop their stock which made for a great flip. Undercutting was very rare, most people respected and posted at the current market price.
A week later, Herbs market tanked (including Anchor Weed which had been selling around 60 g and now is in the 40s) with people emptying their bags into the AH. Sames goes for Ores and skins (except Blood-Stained Bones). I had been doing well flipping Anchor Weed, but now people are posting heaps of it non-stop. The plus side to this is cheap inks for contracts (also crashed but still making some gold) and war scrolls. I suppose the servers which merged with us either didn't see much activity on the AH or there were a multi-boxers (something we didn't see much of prior).
Okay completion is good and especially if you don't want to farm. I am still making contracts, war scrolls, bladders and mount shoes along with the stirrups and hoof-plates. Coarse Leather Brading are worthless selling around 70 silver. Even still selling Uncanny Weapons for some gold (not as much as before). So there are more people buying which is good. But there are the people are selling...undercutting is out-of-control. I'll post Anchor Weed or Azsharine or Runes and come back 10-minutes later and there are 5 other posts each for 1-silver less. The worst though is cut gems such the Deadly Lava Lazuli and Quick Sand Spinel both of which sell great at over 100 g. Post a small batch and come back later and again multiple 1-silver undercuts and then someone decides 'let me get rid of my over-stock for silver on the gold'. So now we have 50+ priced around 30 g. Seriously, you are selling the cut version for half the price of the uncut. I've been tempted to reset, but with all the undercuts and having to re-post the deposits would eat out most if any profits. I should be surprised when Osmenite Ore is selling for less than 10 g. I've heard these horror stories from folks on other realms, but had never bear witness to such a travesty unit now.
I loved War Front turn-ins, good gold to be had posting after server reset. Used to be small quantities posted and cheap enough to flip (or I could buy mats cheap and craft). Now again it is people posting mass quantities and tanking the price. Great if you need to buy the items for your alts, not so much if you are trying to flip. Akundra's Bite was one of yesterdays turn-ins prior to merger was selling at 10+ gold, bought over 3K yesterday at 2.80 gold. Made tons of ink (most of which hasn't sold, so will likely use it for contracts and war scrolls).
I am not in the hunt for the Longboi, but I feel for those on the Silvermoon-Mok’Nathal server who are making their gold via the AH and are still n the hunt I have needed to adjust some of my selling habits including vending some items that use to auction as the posting deposits will quickly overtake whatever profit there was.