Weekly sales?

xD

The problem with sales is that people stop buying if they know there possibly maybe a sale in the future. Look at Steam if u want to sustain sales u have to discount your game as often as possible because it's not selling well between sales

Maybe you can have sales depending on how many items or the prices of the items you bought before!
Anyway, I wouldn't put sales until there is enough material in the shop. But it doesn't sound like a bad idea for the future.