Re: So how long before ...
The selling price of the first thing I bought is not affected buy the buying price of the second.
It is if it's fungible ;)
A litre of petrol bought a year ago and stored in a can is worth the same as a litre of petrol just drawn from a pump. It takes several years for petrol to go off and until that happens a litre of petrol costs what a litre of petrol costs regardless of where it comes from.
You could be honest and have the price vary minute by minute but capitalism isn't a fundamentally honest economic system. Prices tend to go up faster than they tend to go down.