The problem with doing this is even if you are able to find someone who owns a plot of land who is willing to do it on this basis, then you have absolutely no security for your business.
For example Mr land owner says "sure fill it with cars mate and just chuck me £50 a car sold"
Now you go out and stick 10/15/20 cars on the pitch, business starts ticking along nicely and then Mr land owner changes his mind and says he wants his land back ! Where does this leave you ?
If you are going to do it properly from premises then you really need some security by way of a lease agreement, it is possible to get a flexible lease arrangement with most landlords especially on industrial units if you need flexibility.
As said earlier though unless trading from home is causing a nightmare with lack of space / problems with neighbours / wife threatening to leave you etc etc then i would stay as you are and earn as much as possible while you have low overheads.
Selling 2 or 3 cars a month from home may well make you as much net profit as doing 10 a month from a pitch.