That's the exact reason why a PK shop cannot exist without some restrictions in the wilderness. A PK shop could work if you were to kill assigned targets in the wilderness. But as long as we have about 30 players in the wilderness, that target system isn't going to work out.
On-topic: Ancient energy appears not to be popular. Whenever you die in an ancient armour set, you drop less ancient energy than the actual amount of charges you have, so in theory it shouldn't be worthless.
Its worthless because its a pvp set, it's used to pk and one does not simply use one set long enough for need to recharge because he either dies or it simply doesn't need recharging.
Amongst pkers where pvp sets are commonly used the most, there is no need to recharge a set with ancient energy.
Even if you make it eat more charges, ancient energy wont become useful because there's already so much of it ingame. It will continue to be useless that way regards to getting it as a drop.
edit: I kinda understand the idea of not getting a set, but charges. But what if player kills another one in pvp set and has no intention to use energy himself. There's nothing that particular person can do with his drop, which is ancient energy. It's not useful nor it is a good drop because the energy market value is shit.