Stores marked with shopping basket on the map can be 'held up' simply run it with or without a friend, put your weapon of choice at the cashier and watched them put 1k in bag and throw it at you. HOWEVER if you go in with someone you don't know, be very cautious of who picks the cash up. As however picks it up has the option to split the take after you lose the cops. I do it with a friend so no worrying there.
and selling a car is easy, steal one, drive to Los Santos Customs and select 'Sell' from the list of options.
HOw much you making off a TDM?