ok, i worked for a large bodyshop in 2004 when i came into a small amount of money that allowed me to play about a bit buying cars i liked( cosworth/rs2000 etc) and selling them on.
i was buying from ebay(when it was a good place to buy from),which meant me going all over the uk to collect cars. work began to get in the way so i decided to bite the bullet and go on my own.
i rented a small unit (£300 per month plus rates/bills etc),bought a 2nd hand ramp (£600) and began to trade from there. it was good because i spending all my day now playing with cars i was interested in,selling them on and making a profit.i began to get well known for selling well priced cossies/rs turbos/rs2000's so i began to get busier so i could buy more stock etc and even ventured into breaking a few.
all went well for a few years but the rs fords began to go a little out of fasion-evo's and imprezas were getting cheaper and the younger buyers were now buying saxo vtr's etc instead of rs turbos.i tried to follow trends and went more towards the newer cars(which allowed me to own some very nice cars),but it wasnt as good as the ford stuff.
over the last couple of years trends changed again and the car selling game has become very difficult-by this time though i was in a larger unit on the same estate (£800 p/m overheads). cheaper run of the mill cars didnt bring in the decent profit to cover the bills and pay me a wage.
things got harder and harder and i watched what money i had made begin to disappear in bills/outgoings-then my mrs got made redundant-which swallowed a bit more money until in october last year it had all gone. i closed down and until a month ago i was lost-didnt know what to do or which way to turn.
i now work in a local garage on a half decent wage. the days are flying by and im guaranteed money at the end of every month.........

so just be careful. things can go well (then trust me,you WILL spend more,bigger mortgage etc),but they can also turn on their head,like they did with me.

i hope this gives you a bit of an insight.good luck if you decide to go for it-plenty do make a success of it!