As far as I am aware, there is no need to have a licence to buy and sell cars - but as far as trading standards are concerned, as long as you reveal you are a trader whilst advertising the vehicle for sale, and have all the relevant traders insurance that should suffice. I have been told that the insurance for selling cars is quite high, and depends on whether you are working from home or from a forecourt.
I have often thought about trading in cars and recently purchased an ebook about it - which I wasn't overly impressed with the amount of info for what I paid, but I can e-mail it to you if you'd like.
It gives you a basic understanding as to what is required of you - so you might find it useful.
Although my passion is cars, I'm not the most mechanically minded bloke, and as the best way of buying cars to sell on is at auction, I have not gotten the guts to go out and do it.
Also, with the depreciation that most cars have got nowadays, I have decided that to make a living out of buying and selling, you need it as a secondary income, unless you may have plenty of financial backing to start off with. The profit margins can be quite tight from what I understand in order acheive regular sales.
Hope that has given you some insight into the realms of trading in cars - and no doubt good money can be made, but only if you have money behind you to start you in good sted.
Good luck, and let me know if you want the ebook.