When I was 17 I used to buy bundles of vintage dresses from the states for about $60-$70 for 10 dresses plus $30 postage, break them, then sell them on again in the UK for an average of around £25 a dress. A few made over £200 because these things had no upper limit, it wasn't like that dress could be found anywhere else. My ex was a computer geek who used to buy old console's at car boots for £5-£10, clean them up, get them working if needed, then sell on ebay for £45 or so. Again, something that was of limited quantity due to the age and condition. Even my mum used to buy random hats and dresses on ebay, pair them up then sell them as wedding outfits for 3 or 4 times what she bought them for. This was more about people getting a perceived bargain, as a dress and hat rental in a shop might cost £250+, but a used set was costing £100 and they knew they could sell it on again afterwards.
Don't forget if you are buying specifically to resell then this is taxable income, and legally needs to be declared to inland revenue. If you have no other job you may well find you are under the income threshold, you still need to declare you are doing it, but you won't pay tax.
It really isn't easy to make money on ebay now, I had my ebay seller account blocked, with 100% positive feedback, because my dispatch star rating dropped from 5 to 4 stars (all my listings state I post once a week, but people marked me down for this anyway). This month my mum has had a clearout and has made just over £400 on ebay, but owes £48 in fees so this has to be factored in. You need to find a real niche that will sell consistently with a comfortable profit margin to allow for the people who say it got lost in the post (proof of postage means nothing to paypal), or that the item was damaged on arrival, and still make it worth your time doing.