someone who steals an item then sell it on the street for half price.

A person who steals items from retail stores only to then sell on the streets, usually for a discounted price.

A student who takes easier advanced classes so they'd have an easier time getting a high grade in that class.
Primarily done with AP Calculus AB and AP Calculus BC, or one of the science APs(such as taking AP Biology instead of Chemistry).

Highly depends on the school curriculum offered, grading systems, and prerequisites.

Also see GPA whore.

Someone who steals for a living. Usually cars, clothing or jewelry.

In Modern Warfare 2, a person or group of people who use Tactical Insertions to their advantage, go to a rarely-used place on a map, set the insertions, then continuously kill each other to get a free nuke or a bunch of free headshots. This is considered complete and utter cheating and is commonly rejected by professional players. MW2 pro "Sandy Ravage" has a series on YouTube called Booster Justice which is him killing boosters to no end.

A word for a car thief

"The booster" is what is left when you get down to the last 2 inches of malt liquor in a 40 ouncer, preferably Olde English 800 (better known as "Eight Ball"). By the time you get down to the booster it is usually warm, which only serves to make an already unpalatable beverage taste even worse. Despite the nastiness of the booster, it must be consumed to avoid ridicule by one's peers.