I've done (and continue to do) a fair bit of this wherever possible.
My advice... Don't give up the day job!
It can be a useful way of supplementing your income a bit, or making your hobby a bit cheaper. But even if you get hold of "killer" deals on stuff everyone wants, chances are that give it a week and everybody else is selling the same products too, and suddenly the general public knows to expect to pay stupid prices too... Ergo, your margin is shredded instantly...
Stick with brands (as already mentioned) that protect their prices better than the likes of Shimano, and you can make a few quid ok. Trying to make money on Shimano, or other similar OEM specced brands, is financial suicide though.
Oh, and cash is king... Margins are going to be very tight I'm afraid, don't expect to reasonably return much more than 10% gross IMO as long as you're turning things over quickly... So start with £1k, and you may have £1100 by the end of the month. Which isn't feasible as a living. Start with £100k, and you could turn it into £110k by the end of the month, which is vastly more feasible (though a lot more work too obviously).
Chain Reaction for instance, don't make significantly more profit than many smaller companies do. They turn over huge numbers of products at very tight margins though, so trying to compete with them you've got to have your wits about you and not try to tackle them head on (unless you're prepared to invest hundreds of £millions into stock!).