I have yet to find a Klingon Store who buys from player characters. Is there one?

I haven't found it yet.

The female Klingon vendor, Quo'Nos, exchange room.

Thanks. Deep down, I knew Klingons used trading. Bloodworms ain't cheap.

There are several actually, any vendor that sells items for energy credits can be sold to. there are several in the shipyard and a couple in he ground gear badge room, you can even find some on the pvp station. Of course you could just beam up and use your replicator.

Your ship's replicator will give you about 80% of what vendors pay.

The two I found were the Personal Equipment Dealers in the SW corner of the ground equipping room on Qo'nos.

pretty much any vendor in the shipyard on ganalda will take what you don't want/need off your hands

Qo'Nos has at least 2 that I can recall of the top of my head: In the room where you hand in the PvE badges there's 2 Klingons at the far right upper level that you can sell to, they're the ones wearing the trenchcoat kind of clothing.

The shipyard has 2 Klingons also you can sell to, they're to the left when you enter the room there, again they're the 2 wearing the trenchcoat.

On Genalda station there's at least one NPC in the shipyard you can sell to also, I think there's even 2. They're to the left of the room, and if I'm not mistaken, are again the same looking NPCs like on Qo'Nos.

Hope this helps