My first main concern when it comes to buying assets is compatibility. They have to be compatible with other assets I'm using, period.
I need a complete set or I should say plenty of assets but most of the time, they are too different to each other.
My second main concern is suit my design. Ex: I want a dragonian who could attack in 5 directions. If an asset pack is missing just one direction attack sprites, then I won't buy it. I know I'm very picky but I don't want to compromise my design just because the asset is missing required sprites.