They tend to package and label orders as they come in, that's the only thing you can do to be efficient -- you can't let them build up.
But then they only drop off (or get pickup) 2x/week, e.g. Tues and Fri. Which might be fine if that's what their shipping times indicate.
But then the buyer gets confused because they assume it was mailed immediately, which it wasn't. But there's no way for a seller to print shipping labels from eBay in advance without eBay marking it as "shipped".
It gets even more confusing because with bulk pickups or dropoffs, they often don't even get scanned when the carrier receives them. They won't show as actually in the carrier's hands until they reach the first major hub, which can easily be a day or even two later.
AIUI, Amazon's policy is that credit cards don't get charged until the order ships.
As such, some shadier folks will create the label long before the item is actually shipped. However, since the label has been created, the order is now marked as "shipped" and the credit card is charged even though nothing has been packed, let alone actually shipped.
I assume they do this to avoid complaints of slow delivery when the sender takes a whole to post the item.