Great job! Does it work when placed on a static mesh, or Landscape only? I suspect that it works, since you use the DistanceToNearestSurface, am I right?
Also, do you assign a material/texture to use for blending for every mesh, so e.g. if you place a rock on a sand, you assign sand texture for blending, and so on…? Or does it detect ground texture automagically? 
Sorry, my knowledge is limited here, I’ve never played a lot with Distance Fields…
BTW. I’ve tried to make something similar for my game, but in my case it needs to be dynamic since my world generates procedurally - and I’ve stuck here, because I can’t figure out how to e.g. detect ground texture below procedurally spawned rock + what if it spawns on top of two different textures (for example over some grass texture <-> mud texture intersection), etc.
About Marketplace: I would buy it definitely, hereyou can read about submission guidelines. Just prepare the required folder structure, add some documentation/level with examples + demo video and you’re good to go 
To submit your pack, you can start here: https://publish.unrealengine.com/
Awesome WIP, keep it up!