These are just a few ideas: We can sell nether Wart, We can sell Tree Leaves maybe, we can have /ah which stands for auction house last but not least maybe sell Golden Apples. just a few ideas
You can already sell netherwart by doing into shop picking any category and clicking the sell hopper and dragging them in
To sell your nether wart you can go to /shop under Farming which is the seed and right click to sell one nether wart or shift right click to sell a stack , they have just recently been increased in sell price from around 3 to 15 .
out of curiosity. Why not have ability to sell things that can be sold can be sold no matter what shop window you open? Why you need to locate the correct shop first? edit: I mean you could open farming and sell in there your diamonds, farming things, others that can be sold.