Enchanter spells in HHK
 

Are DE's KoS in HHK?
Does my enchanter need his Illusion:Human spell to buy his Pet Spells?

I don't think Agnostic DE's are KOS, but you can invis to the merchant (it's on Sol Ro temple faction, so you can buy regardless).

If you walk into W/E Freeport with that human illusion on and not be attacked, you're very likely to have the same reaction in Highkeep...but yeah, invis works too, the vendor doesn't have any guards in his room.

Buy ahead if you can, and remember... charmed pets do more damage than summoned ones if you have good charisma to reduce early breaks :p

I believe all my DEs were KOS there until I did some faction work.
Invis the easiest option.
I believe any good race illusion would also work.
Perhaps casting Alliance/Benevolence on the first guard you see as you enter
the zone would work also, but not sure.

As stated, those two merchants are alone in a back room without
any guards near by. They sell to anybody.