Find My iPhone needs to be disabled by the seller first, reset factory defaults does not fix this (otherwise if your phone was stolen someone could simply reset it and disable the “find” feature.. which is pretty silly).
Contact the seller, ask them to login to icloud.com using their details and remove the phone from their list of devices.
You should be able to see the email address of the previous owner. If the phone has been stolen then the seller may not be able to reset this for you, and you should return the phone for a refund.
There are a few available here on CodeCanyon. Some are standalone systems separate to WordPress, others are WordPress plugins.
Use the search tool for “invoices” and filter the results on the left hand side ( e.g. http://codecanyon.net/search?utf8=%E2%9C%93&term=invoices&view=list&sort=sales&date=&price_min=&price_max=&rating_min=&platform= )
Are there any people which use codecanyon as their “main” working place to earn money?
Yep! (actually there are quite a few people who do)
And can somebody give me some advice for the future?
Keep working hard, close down any distractions when on the computer (e.g. facebook), and if you have an item waiting for review then start work on your next item. Try to think of something unique, solve a problem that not many people have solved, or solve it in a unique/different way to others.