Some ideas for you that might cover various uses of indie game genres
(popular usually for TDS/platformers)
2d cartoon style pixel arts pack with polished graphics of
- zombies, skeletons, monsters, etc
- snipers, shooters, etc
- melee fighters like knight, swordman, ninjas, beat em up, etc
- ranged chars like archers , wizards, etc
- kid surfers, skateboarders, thiefs, runners etc
- gems, jellies, candies, etc
- effects, explosions, etc
- cars, racing, tanks, etc
- and etc.
From what i observed from multiple graphic asset stores and also from my experience made a purchase of this kind of asset there before, i think what these graphic asset buyers need are great polished arts and is packed with multiple variations of contents (male/female/fat/skinny and idle,run,walk,front,back,side,jump,attack,defend,miss hit,being hit,death, etc)
I think for non-exclusive assets (mass production like on marketplace). The price should be affordable too aka indie budget friendly. In general, a "pack" of 3-4 chars with complete sets of animation sequences (not a single sprite) is about $5-15 as i can see from popular marketplace (best sellers), or even more if it is exceptionally splendid (AAA), but the sales might be for a limited audience. The graphics are usually offered editable (vector,psd) and the license is usually for single one title game only. Multi-license use need more license purchases. That's what i know from the graphic assets market industry out there.
Just visit graphicriver / other graphic asset stores and you will see/learn what graphic assets are best sellers. You can see @ graphicriver / other graphic asset stores etc, for example which always lists amount of sold by popular seller.
And i can say, as of now, the YoYo marketplace is lack of those assets. It's good to see more artists want to produce assets on YoYo marketplace, so we don't have to go "far" there because it is also available "near" here, as long as the quality can compete with the assets there.