you have to be on the same level as the vendor to buy them . the work around is put a petball in a bag and sell the bag.
i have noticed some places (ilshenar i believe) your pet wont come out of the stable when called by a petball . you can however claim your pet , tell it to stay next to the stable , run to your target location , use the pet ball , and it will come to you.
you can recharge petballs with powder of translocation like you do with bag of sending. however , like bags of sending , pet balls also can be depleted . once they are used so many times you cant recharge them.
you have to go into the solom hive to get pet balls . you don't need any specific quest . just kill the queens . however it can take alot of time to get one to drop . if your gonna be in there a while you might want to do the matriarch quest to get friended (nothing but the giant beetles and ant lions will bother you)
.... and once you're friended you cant attack anything in the hive other than blue beetles and ant lions or you'll blow the friending . even an autodefence slap when you step in a green puddle will blow the friending.
this makes hunting the queens rather dificult if you cant attack them .
what you do is take a nightmare or cu sidhe . ride it into the hive , run up to a queen , hop off it and say all guard me (to get it into war mode ... you wont be able to loot if its not) wait for the queen to attack the pet .
its a rather long drawn out process and some would rather not bother . so its your call . buy the balls . or go get your own .
personally i get my own . i also redo the matriach quest freequently to get powder ... the spice must flow