It would be wise to ask the shop if the RTR car you want is legal to race.
RTR means Ready To Run and most of the time that is a car with pre-painted body, radio, tires and and engine. Batteries, fuel, tools, ignitor and spareparts are not included.
When buying a car you have to run-in the engine, that can take 1 or 2 liter of fuel before you can drive with maximum power.