I used to buy things in bulk and then sell them in smaller parts. When Pandora first took off, I bought a large lot of replica beads (never advertised them as real, but people were looking for a cheaper alternative) for £5 and sold them one by one for £30 total. Then I spent that £30 buying more etc etc and made quite a lot.
I also bought other things because the beads would sell around £10 worth a day, so when I had a little more to invest they weren't selling fast enough, so I got things like the bracelet strands (again, replicas, got 10 for £10 and sold them for £2 each, and bought more when I was in profit).
It was quite a fun summer actually, but then eBay put their prices up so it wasn't really worth the effort anymore, because of the time it took to go to the post office (I don't drink and it's an hour round trip that I was having to make daily) and I had to stop when I got back to college because it was taking too much of my revision time.