While popularity, condition, and many other factors are at work when determining a card's price tag, sometimes nailing the color choice and pose makes the biggest difference
In general, I would look at these two factors when looking to include cards not of your tribe's creature type: Is this creature so strong that it, on its own, is generally stronger than my tribal creatures when they are receiving bonuses and/or empowering my other cards
It merges whites mass destructive power with blues master spell knowledge to make an end-all be-all board wipe
TCGPlayer Pro (Quicklist) offers scanning and listing tools through the TCGPlayer Seller Portal, but it's designed for TCGPlayer marketplace sellers rather than eBay sellers
Finally, it gives us some answers to cards like Whip of Erebos, albeit only partial, crude answers