To make holding up stores easier you can buy a mask from the beach area and equip it whilst you hold up the store. Once you have taken the money and are evading the police you can remove the mask (hold select on the ps3 > inventory then mask) not sure what it is on the xbox. If done whilst the police are searching for you, but not activley chasing you you will lose 1 star. Handy for getting rid of helicopters. This can also be done the other eay round - rob the store without the mask then equip it.
Also i wouldnt purchase the 400k apartment, from what ive read all the 10 car garage properties are exactly the same (Garage and apartment) the only thing you pay extra for is the location and the view.
And as someone else stated you can sell a car every 24 hours in game (48 mins).
