do a quick gurgle search for 'affiliate marketing' and you will be able to find out more than you ever wanted to know! There are many different ways to do this type of endeavor and I have not looked into the one that got all this started but I can verify that 'affiliate marketing' is a legit way of making money... for some.
Every one of these forms of business I have seen do have some startup costs, this can range from a few dollar up to the sky. You will be confronted with a host of expenses from choosing a web hosting service to 'search engine optimization' decisions to get your web pages listed favorably on search engine pages. All of this is very much 'for sale' if you have the money to put into the pot. There is NO SHORTSAGE of affiliate marketing gurus out there that make a living by just touting their own 'expertise' on affiliate marketing... I can verify this with just one example, right now there are over 300 emails from just ONE of these gurus sitting in a folder in one of my email accounts... this is from a guy that I only made an inquirely to and never did any business with at all... 90% of these emails (of the ones I actually have read) are nothing more than soliciatations for me to spend more money on his 'new gimmick'. And YES, I would get a nice fee from sucking other folks into his schemes as well as a piece of the action each might produce.
Now let me be clear here, there are a lot of good and honest ways to make money in affilate marketing, but it is NOT as easy nor as simple as just wanting to do it. You can offer a million different products that you never have to inventory or see. The key is to have the knowledge of what works where, and the funds to change your program when you meet with failures (eat the losses) and keep going until you find the right mix of product, marketing, and knowledge of it all to expand your business. This is not for the faint of heart or those trying to shoestring a venture into mega bucks.
Just my thoughts here, but I HAVE explored this a good deal beyond the surface level.
Best of fortune to all!