My rarest system "was" my Sega Nomad which I bought at the Goodwill for $5 and resold it on ebay with a few games for $90. I kinda wanted to keep it...but something came up and I needed the extra money pretty badly.

I almost became the proud owner of a PC-Engine Duo RX system complete with box along with Dracula X: Rondo of Blood. My cousin had it and was going to sell it all to me for a measly $200. However, he then decided he didn't want to sell it. But he did let me borrow it for a couple months along with Dracula X.