Hi im only 15 but when I want just a little extra cash, I usually buy a mobile phone and then re-sell it for a profit.
I look for a while to see the average prices that each model sells for, then if I see one going cheap, grab it quick.
The last one I bought was a D500 which I bought for �54 (including postage) and resold it for �73 (incl postage). As it cost me �4 to post I made a profit of �15.
I do find it time consuming however if someone could be bothered to do this many times then I think it would be worthwhile.
You can also try to search for mis-spelt items which generally sell for less as they get less attention. To search for mis-spelt items then this site is quite useful:
http://www.goofbay.co.uk
Good luck!!