Cactuspete just FYI, how here in D a binding contract is formed (and this true true in most not common law countries too).

Step 1: The seller tells you a price ("invitatatio ad offerendum"), which is the price list displayed here.
Step 2: You offer to buy for this or a different price.
Step 3: The seller accepted the price and confirms the buy (done here by the confirmation number) or refuses to sell it (gives no confirmation).

After these three steps a binding contact is formed, the seller may still call for a "Erklärungsirrtum (declaration mistake) (BGB 119)", but has to do so without any delay (BGB 121), which was not done here, since over one week is by no means immediately.

CP I hope this explains my reasoning.