My favorite thing about eBay is the simplicity of everything. To get started selling you just setup a free account, enter your title and description, click a few buttons and whatever you have to sell is up on one of the most trafficked sites online.
eBay puts the buyers infront of you. People who go to eBay are there to buy things, they are shopping, looking for a bargain especially in these economic times.
Most other online business startups would require you to invest lots of time or money into getting traffic to your website, were as eBay has already done this for you.
The biggest and really only hurdle there is when selling on eBay is making sure you're selling what people want to buy. It makes no sense to list things no one is paying for, and this is easily discovered with a simple search.
Let's say I see a certain cell phone on eBay that's really popular. I do a search for it, I scroll down and look on the left and see "Search: Completed Auctions" I click that, and I can instantly see what's selling, who's selling it and for how much!
I mean, it doesn't get much better than this. You already know within seconds how much profit there is if you can find a source for the product.
So, maybe you find something and it's selling like crazy but you don't know where to get it cheap, easy just do a search on eBay for "PRODUCT NAME wholesale" or "PRODUCT NAME wholesale lot."
Just like with any business or physical products there's the manufacturer, the distributors and the end user (buyer).