That link Ricjhelle has given has some good reading in it, but I would suggest that you may consider doing how I started, second hand!
Weekend markets, garage sales, these are little gold mines. I started with about 100 dollars, very long time ago now, but I turned that initial 100 into a few hundred and never looked back.
I never went away from that supply source, built up to a power seller inside 3 months of joining eBay and it was all items I sourced through garage sales.
All I did was open up other second hand supply avenues, such as weekend markets and going to auctions, insurance, deceased estates and so on. Honestly, nothing has changed on eBay, if you do your research as to what is getting money on eBay and then take 50 or 100, be able to talk a good deal, then you can do very well on eBay.
If you do go down that route, develop a newsletter, because I found that I had a high level of repeat business. I rarely had the same item twice, just the nature of second hand, but it was the price and the variety I had that brought customers back to my store over and over again.