I very much agree with this. eBay's website should be a 100% SSL experience at all times while logged in and never redirect out of that until logout. I can see while being logged out not having SSL as there's no account infrormation or the like being exposed.
The problem is that this would create a lot of overhead that eBay is unwilling to take due to the increased processing time required by front-end load balencers and similar systems to perform the additional encryption. This isn't an excuse however but is a possible sign that eBay's infrastructure may be underequipped to handle a 100% SSL experience or an issue whereas people may complain that the overhead may slow each page load. At the same time I believe that solid SSL should be provided at the minimum as an option for those who don't mind the delays as an opt-in..
Doing some tests from my internet connection over 3 tests each from my Firefox browser from signin.ebay.com (no different than regular web requests, just with an attached "stop watch" if you will)...
Without SSL: 0.179 - 0.195 seconds per response
With SSL: 0.343 - 0.478 seconds per response