I believe another advantage of using eBay's system is that $100 of free insurance is provided for Priority Mail shipments, vs. $50 of free insurance if you don't use eBay's system.
My wife and I continually sells a variety of relatively inexpensive (mostly under $30) items on eBay, and we live in a high-rise seniors condo building that has a keypad entry. We leave the outgoing packages on the floor by the bank of mailboxes for the entire building for pickup by our USPS carrier. I have made friend with our regular carrier, and some time ago I asked him when outgoing packages first get scanned into the USPS system - he responded that he does this as soon as he return to his truck, which is reassuring. Still, when, we ship something more valuable, roughly $100 or more, we take it directly to our local branch post office and watch them scan it, just to be extra safe.
Alan, re your question as to how much savings one obtains by using eBay's system vs. hand-writing the shipping label and taking the package directly to the post office, for Priority Mail packages, it depends on the weight and distance. For example, for a 2 lb. package sent Priority Mail to Zones 1 & 2 (not very far), the USPS' retail price is $5.80, whereas via eBay we pay the USPS' commercial plus price of $5.09, which is discount of $.71 or 12.2%. For a 2 lb. Priority Mail package to Zone 8 (far West), the retail price is $11.25 vs. the commercial plus price of $8.95, which is a discount of $2.30 or 20.5%. Here's the link to the USPS' price lists (formally known as Notice 123):
http://pe.usps.com/text/dmm300/notice123.htm
Also, delivery confirmation is free when using eBay's shipping system. It used to be that the shipping discount gave eBay sellers the opportunity to make a bit extra on their eBay sales, but then a few years ago, eBay and PayPal got smart (or greedy) and began applying their commissions to the seller's shipping charge in addition to the selling price of the merchandise.
FYI, one can also establish an online account directly with the USPS and then create & print shipping labels for packages, except for First Class packages (which one can print labels for via eBay's system). One does receive a discount from the USPS for doing this online, but I believe the discount is less than doing this via eBay, and I don't think delivery confirmation is free (I seldom have occasion to do this, which is why I'm not certain).
Val