The Soul Trader is one of the more rare and valuable pets out there so in order to not get robbed you should probably ask for a bundle of pets in trade for him.
Thed question on what pet could help you in your battles depends on your playing style, if you do PvE or PvP, daily trainers etc...
When trading a rare pet I'd say go for something you really like, so that you feel it's a good trade for you.
I'd love to get a Soul Trader and would happily trade a bunch of pets for it, I've got many TCG-pets, Darkmoon Rabbit, all Pandaren Spirits, some rare ones like Searing Scorchling, most Raiding with Leashes pets.
Just check my collection and let me know!