Normally you will see on the description of the item that the seller undertakes to post within so many days - mine says "2-3 working days". How do you know that it hasn't been posted? Not everyone tells you - I always update the "despatched" option on my Sold view, plus I email the seller to tell them that it's on its way, but not everyone does.
If you only paid on Wednesday, then it's possible that the seller hasn't been able to get to the PO - I find this does happen sometimes as I work fulltime.
It wouldn't hurt to drop them a message today asking when you might expect delivery, as they haven't contacted you at all since you paid.