Just released an article on Second Life commerce programming.
Commerce occurs in Second Life whenever money changes hands. Money, in Second Life, is the Linden Dollar. Although the Linden Dollar is not an actual currency, it can be freely exchanged for real-world currencies. Like many real-world currencies, the value of the Linden Dollar rises and falls against the US dollar. As of this writing, 1 US dollar buys approximately 186 Linden Dollars.
Writing Commerce Scripts in Second Life