1. When you sell astro items back to the fashion shop by dragging it into the "Sell" box, you'll receive points in return. These points can supposedly be used to buy astro items, but OGP didn't implement that yet, so the points are pretty much useless for now.
2. Some astro items are tradeable (like darts, puzzle coupons etc) and people are allowed to sell these. Basically, if you're able to trade an astro item using the trade window, it's allowed.
This is based off of my knowledge of the game, and anyone is free to correct me if i made any mistakes