I admire your balls for attempting to open a 30 car pitch with no prior experience.
It can be done but it is difficult. I personally think the hardest part will be valuing the part exchanges etc. I think you will come a cropper if you just look in glass or cap and say oh it must trade at this. Some cars are over book, some are considerably under.
I personally would go and sell cars for someone for a minimum of 12 months to learn the ropes. Not a main dealer where you are a glorified meeter and greeter but at a place where you can learn the true values of second hand stock.
Good luck though