Average pricing implementation in a POS application?

I am developing a POS (Point of Sale) application which should be able to calculate average price. Here is the deal, i don't know what it is and how it is calculated for consumer products. I just got this requirement. All i know is this, that there is no way to know from which particular stock of goods, an item was sold from (i.e. when stock keeps getting replenished). Has anyone done anything similar, please help me out on this. or if you could suggest me to some resource.

I am not very good with b开发者_StackOverflow社区usiness vocabulary so please ask me again if there are ambiguities in this problem statement.

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I would ask the customer. Asking on a programming Website sounds like a really bad idea!