If you really want to earn extra income through internet marketing, you should confirm what you sell is really good and quality products, should learn many knowledge about business and you can find 2 or 3 partners with you, together do some, everyone earn money together is better ways to expand your internet marketing.
Not necessarily. You don't need to be the seller to do internet marketing. A seller, site owner can hire you to advertise his site/product for some Bitcoins. It's profitable for who have marketing skills and can promote others services well. There are some strategies that can be used to make the advertisement more interesting and flashy.
Yes. All you need is to promote some products from a reputable affiliate network. Not necessarily a bitcoin network like RUNCPA, you'd have a long run before you can sell some of thier product because you have the limited audience. ITs best to stay on Commission Junction or Peerfly. Also stick to the forum where users get to have a better advice not like bitcointalk.
Forums like warriorforum and blackhatworld where users are much knowledgeable on what to do to get a sale, they may teach you some tricks you never have heard before for CPA.