Here's something I discovered that may or may not prove useful to someone else in the future.
I made a Buy It Now purchase on ebay last night during a cashback session that should have been cashback eligible. However, I could not pay immediately due to waiting on the money to become available in my checking account (the price was too good to pass up even if I wasn't able to get cashback). I had to wait until this morning to pay, but just to see I went through a cashback link to get to ebay, which put me in a new cashback session. As it turns out, despite a gap of about 10 hours between purchase and payment, as well as being in a new browser session, I still succeeded in getting the cashback applied.