How you find the stuff to sell on ebay is up to you. Look in stores, shuks..etc. Use any contacts you have (friends who own a business that makes something) to find a wholesaler or factory who can get you stuff cheaply. If you are a good bargain hunter, you will find stuff in stores selling for cheap that you can re-sell on ebay. If you are creative, you can make your own stuff.
I'll say the obvious thing here; you can get into a lot of trouble selling fake purses, fake clothing, bootleg computer games..etc. Ebay can shut down your account and Paypal can hold any money you've received. And it's tacky.
The most important thing is to do your research first. Don't list an item if you don't think it will sell... it's just a waste of time and money. Don't list an item for such a low price that you aren't making any profit... also a waste of time and money.
You can also set up a “we sell your stuff on ebay” business. If you have friends/family that want you to sell something on Ebay for them, it's a good idea to charge them for your time (and the listing fee) up-front and then take a commission if it sells. If you are making a business out of this, you should get paid for your time and efforts, even if the items you list do not sell.
Most people have no idea of what Ebay fees cost and how cheaply people sell their stuff for on ebay. When they ask you to sell something for them, they oftentimes have overinflated expectations of what someone will pay for their stuff. Be realistic and do your research and give them the market price for their item on ebay... or explain to them that there are sellers who are selling 10,000 items exactly the same as theirs and that it's super-unlikely that their item will sell.