Here are a few things I would like to clear up.
1) Anybody can open a shop. They may sell items, jutsu, summons and elements. They may only sell summons and elements they own as well as jutsu. They can make equipment and sell them if they wish even if they do not use it themselves. Try to sell things reasonably; you're not gonna sell a C-Ranked jutsu for a high price are you?
2) You are allowed to to buy and use jutsu even if it isn't your elemental mastery but you are not allowed to create the jutsu element unless you purchase the element itself. You may not be able to purchase a new element unless you have an extra slot for it. This goes for summons and familiars as well.
3) You are allowed to try to rob a store only when the shopkeeper is online. If you win the fight you will be able to take some of the shopkeeper's ryo and maybe an item or 2. Of course if the shopkeeper refuses the fight you may take what you want freely. This is how much you are allowed to steal from the shop if the shopkeeper has the amount as well as items.
Ryo allowed to be stolen if fight is won.
Genin : 500-1,000
Chunnin : 1,000-5,000
Special Jounin : 5,000-10,000
Jounin : 10,000-15,000
S-Ranked Jounin+ : 15,000-20,000
Items amount allowed to be stolen.
Genin - 0 Items
Chunin - 1 Item
Special Jounin - 2 items
Jounin - 3 items
S-Rank - 4 items
Sannin+ - 5 items
If you lose the fight however, well your fate is up to the shopkeeper.
4) You are allowed to haggle and lower the price if you are good with your words or the shopkeeper is nice enough. (Or you become a sneaky bastard and put the shopkeeper in a genjutsu.) But if the shopkeeper doesn't like it, he'll throw you out.
5) And above all remember to add the items and things you buy into your profile and have proof of purchase by showing the topic you purchased it in. You may not purchase and item for your alternate account.