1) First is the fact that you will need a website. Although you can certainly make a great income just selling from eBay alone, the fact is that for every sale they take a percentage of your sales. If you were to sell from you website though, now you are cutting one cost of your business, which will definitely help you in the long run when you have a customer base!
2) You also need to be organized! Even though selling online is much less paperwork than a brick and mortar store, you still have to have good organization skills. Why you ask? If you are receiving e-mails everyday with questions about your products, orders, shipment dates, complaints, policies, etc, how are you going to keep everything straight? Keep in mind you could be having on going conversation and it's not easy to remember exactly who is who. You also need to be able to keep track of sales records, such as payments received, shipment dates, feedbacks, e-mail templates, auction ads, inventory, and more. It will take some time for you to develop a system that works for you.
3) One thing people love about eBay is that there is less of a time commitment required depending on how large your business is. But I will say that when you first start out, it can take a lot of work just like any other business. Since you are just starting, you will have to adjust to doing certain things, keeping track of transactions, designing ads, policies, opening accounts, etc. It's not that any of them will be particularly hard, but as you first start out with anything new, there is an adjustment period or learning curve. Selling on eBay is no different.
4) And the biggest one that you need to know, is starting an eBay business takes money! It won't cost you $20,000+ like a traditional business, but it can still take a sizeable amount to start. Imagine paying for a website, buying products in bulk, listing fees, paypal fees, account memberships, and more. These can easily run into upwards of a couple thousand dollars. This will obviously vary depending on what you are selling, but I'm sure you get the idea.
Obviously this list could go on and on and in much greater detail, but I wanted to at least introduce this to you, so that you realize a bit of what it takes to start an eBay business, or any business for that matter! I wish you luck in all of your future ventures!