2. user can see clearly what shop is selling/buying by just looking at shop
3. better and informative error display like:
"Error. You dont have enough items to pay" or "Error. Shop out of stock."
4. if shop owner is admin shop can sell/buy infinite items - no "out of stock" errors
5. shop errors now show as shop text - owner can easily spot shop with problems
6. players can't trade with himself to prevent easy abuses