Looking in the right place at the right time
Which cards should you monitor closely at the moment, and which cards can be ignored for a while?
I wasn't suggesting to buy a lot of Battle for Zendikar singles during pre-sale, or since release. The main problem I saw with the set was that the hunt for Expedition lands would cause an overabundance of the regular rares and mythics, which in turn would depress their value. Once the entire spoiler was released, it was clear that we would have the best mana bases ever in the new Standard. These new mana bases were optimal for decks that were splashing everything, while avoiding cards with double or triple commitments to a single colour. At the same time they are ideal for casting the overpowered Khans cards, which were created to shake up the format after Theros had pushed Standard towards mono-coloured decks. Battle for Zendikar doesn't have many similarly powerful cards, and the few it has are featuring double casting costs in a single colour. The new mechanics in Battle for Zendikar don't mix that well with the mechanics in the other four Standard sets either.
For all those reasons it is not surprising that Battle for Zendikar spells have not had a large influence on Standard. It is quite fitting though that Zendikar (the plane famous for its land theme) revolutionized all the mana bases instead, which caused the massive price increase of the Khans fetchlands.
Speaking of fetchlands, what should you do with your additional copies?
The recent and still ongoing price increases were caused by Standard, as the battle lands transformed the average Standard mana base from using 4 fetchlands to using between 8 and 15 fetchlands. Some players just stuck with a single deck last season and did not own playsets of all five Khans fetchlands. Now they had (or still have) to catch up and complete their playsets.
Clearly you were smarter than that and did not only have your playsets already, but are sitting on lots of additional copies as well. Now the question is, whether you should cash in to double your investment now or wait for them to rise even further. I suggest watching the price graph very closely in the upcoming weeks. At some point the new demand will slow down and the prices will start to stabilize. That is the perfect timing to sell, as it is impossible for anything in Oath of the Gatewatch to increase the need for fetchlands even further. That way you secured your gains at a local maximum and you can invest into fetchlands once again at a local minimum when they rotate out of Standard next spring.
Supply and Demand
Coming back to Battle for Zendikar though, let's examine whether it will indeed be plagued by the “curse of the Khans” just like last year's autumn set. Battle for Zendikar was expected to beat the record for best selling set ever, so how are supply and demand shaping up on MKM so far?
Gideon, Ally of Zendikar Available items: 1081
Drana, Liberator of Malakir Available items: 1899
Ob Nixilis Reignited Available items: 2180
Compare these numbers to the ones of Khans of Tarkir, the previously best selling set:
Wingmate Roc Available items: 1345
Anafenza, the Foremost Available items: 662
Sorin, Solemn Visitor Available items: 1102
As you can see the sheer amount of Battle for Zendikar product offered is already beating Khans of Tarkir by a large margin four weeks after release, when demand should still be high. Keep in mind that players will still be drafting this set for months and the redemption wave hasn't even hit the market yet either. The result is that prices are on the downgrade for almost every card - even Gideon. There will be investment opportunities shortly before the next rotation, but for the next few months I would look everywhere else but in Battle for Zendikar.