The object of the game is to buy a car and a villa in a maximum short period of time. Also, I guess, the objective is to get yourself in the hall of fame - to be recorded there you must have a rating sufficient for that. You are recorded as a hero when you press on the 'Quit' (F10) menu item. Then your current game will end.
Don't forget that every month you pay 5% tax on your car and 2.5% tax on your apartment -so calculate their cost and your means.
In general, if you want to be sure of safety of your car or apartment then you should put your car in a parking lot (100 coins a month) and install a fire-control system in your apartment (300 coins a month)
When you buy another car or apartment your former property is automatically sold for 50% of its cost.
Messages "You can't buy this!" occur: if you can't afford the purchase, if you have a debt to the bank (with the exception of a room, Ford Ka and Peugeot 206, but I don't recommend take them on credit because later you will have problem with repaying the interest), or if you already have such property.
Also if you have a car you can casually work as a lift giver (a Gipsy cab driver) and the sum you can earn depends on the cost of your car. (10 to 80 coins).