The Sales

I love and hate this- Sales.
I bought a Loreal foundation last week from the Loreal specialized store.
And guess what? Today I saw advertisement of guardian. They have discount on this product.
Damn it! It's 10 bucks cheaper!
This is not the first time such incidence happened.
So I'm wondering, if next time, should I wait until there's sales only I buy that something I want?
But there's another incidence, if I wait to have sales, I'll end up buying nothing.
Know why?
For example, I went to vincci and I love a pair of shoes.
It's a bit way too expensive and I just postponed purchasing.
When I decided to buy, I went back to the shop. ALWAYS, they don't have the size of mine already or that's the last pairs of that size which I don't like because it's the displayed one.
Arrghhh.... really hate it!