As an example, during Hol Hamo'ed Pesach, could you buy chametz online knowing that it will arrive after Pesach?
My assumption is that it would be OK, as the buyer doesn't actually own the product until it arrives at his home. Proof of that fact is that say, if you order from Amazon, they are responsible to replace the item if it has not arrived / got lost in transit.
Or, do we say that once someone has paid for the item, he owns it even if he has not physically acquired it?