Your purchase number can be located in two different ways:
If you paid your auction, the purchase number appears at the bottom right of the orange strip, which includes the confirmation number. The purchase number is always composed of 7 digits.
If the auction is pending, you will need to access the "My bids", "Win-Pay" section. Here, then you have to enter the simulation of the payment realization and you will be able to identify the purchase number.