I have accumulated a fair number of battlefield items from the Portal, and since I've already built up all my buildings, I should probably sell them.

What are these items actually worth? 

All I remember is that items like the nightcat and money tree are (or were) worth more than high-level blueprints. And apparently now there's a cove background that also drops from the portal? I assume it's not worth as much as the other things but frankly I'm confused at this point. 

(I did try selling a bit ago, but no one bid. So either no one noticed the listings or I wildly overpriced them.)

I think Stable level 5 and Warehouse 6 are worth the most. However I'm not sure who would be at the point of needing them.

Also cove backgrounds are quite sought after.

Maybe make exchanges for vermillion or premium items to get rid of the snake eyes and so forth?