This seems to be quite a regular scam I don't know if it is successful but after having done some research I have a grudging respect for the operation and the patience of those conducting it - having said that best not to get drawn in and give a polite 'thanks but no thanks'.
Big Idea 151 from Crofton is Supply and Demand
Adam Smith bequeathed to Economists the term 'the invisible hand' it denotes the forces around Supply and Demand.
Supply and Demand along with elasticity were the first serious concepts I encountered when I studied Economics at school ('O' Level many years ago) - to summarise, Good A is produced and sold at Price P as the supply increases to meet the demand of the market an equilibrium is reached where all
|A classic supply and demand curve|
Products such as Salt have a fairly inelastic demand curve - that is the price changes do not affect demand heavily as people buy salt for a purpose and generally only as much as they need (unless they're speculating and that's a whole different story).
I suppose we can apply this law of supply and demand to many things (performers leave their audience wanting more).