Not all eBay sellers always mark as shipped on the items they sell or the items you purchase.
The eBay seller if they are a small eBay seller may not mark the item as shipped even though they have shipped it and not all small eBay sellers will use tracking information either.
If the eBay seller uses online postage and buys the postage with online tracking the information will get uploaded to eBay automatically and the item will be marked as shipped and a tracking number provided.
But some eBay sellers who sell a few items here and their may just go to the post office with your package and pay for the postage without any tracking number.
Then the seller may or may not mark the item as shipped or give you a tracking number even if they do have one.
If the package does not arrive on time or after the estimated arrival time then you can contact the seller and if no response within a few days you can report it to eBay.
Also some small eBay sellers only sell part time and have jobs outside of eBay so even if you order something or win an auction on something on eBay the seller may not be able to get to their eBay account and check the orders and ship them out until after work.
If it's on a weekend or during a time when the post office is closed the seller may still process your order after they get home from work and then print the shipping label or at least have it packed and ready to go the next day if it's a day when the post office is open or when UPS or other shippers pick up.