My list would include the Dior Street Chic Columbus. This was from oh... 2003-ish? I loved the strap. I've seen it recently for sale on consignment sites for a couple hundred dollars or so.
The Louis Vuitton Sofia Coppola SC. Available new or used, but too much $$$ to even consider.
And the Givenchy Antigona. It has been on everybody's arms and comes in so many colours and leathers too!
However, there are no bags that I completely regret missing out on. The one item I do miss, which I had purchased, then returned out of buyers remorse.. was a pair of Tod's strappy sandals from Spring/Summer 2009. (I think I may have written about them before.) They were the same ones worn by Gwyneth Paltrow in that season's ad campaigns and the style would work year after year. They were still too expensive even on sale and now I have never seen them listed anywhere for sale pre-owned. I did take photos before I returned them, though. Yes, I'm that dorky. *sniff sniff*