Perhaps the most important is..what the card means to the person it belongs to
However, I am here to tell you that, in terms of design, Legends might be the most innovative set apart from Alpha
Like most Pokmon cards, the value of Eelektross depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Lost Origin)
If Bruse Tarl leaves the battlefield after other Oxen you control have dealt first-strike damage but before regular combat damage, those Oxen won't deal regular combat damage (unless they still have double strike for some other reason)
You can convert it by removing +1/+1 counters from your artifacts to create mana, which you can only spend on more artifacts