Which items are easy to sell for zeni?
How about the U15 items can I sell them to people for 1000 zeni?

I've never been able to sell my U15 items so I just vendered them all. You might be able to set up shop near a low level quest hub, but I don't think you'll get very much for them.

botfarmers sell 20 blue U15s for 50k, and 20 Pink U15s for 10k.

undercut them and maybe ull be able to.

gimme gimme gimme. except the black ones :)

The blue U15 1-3 ones you can sell for 3-4k each and the purple stones you can sell for 2k each.

There are lots of them so its hard to explain but I sold 5 of a certain kind for 200k, just have a look at the prices other people sell items for.