I'm a big fan of getting rid of the useless junk and redundancies in my collection, but the only things I regret getting rid of are my TurboDuo games. I bought a US Duo new from TZD in 2001 along with Dracula X, Lords of Thunder, Gates of Thunder and a handful of hucards. Those would be my only titles for a long time, and as great as they were I was at a point where I needed cash to pay some bills and I hadn't played them in a while. I sold my boxed, still mint and looking like new Duo on ebay for $180 shipped. I figured I could always rebuy it, they weren't expensive then.
A little while later, still without a system I traded my Dracula X and Lords of Thunder for a PSP and a few awesome games. This was when the PSP was still new, and the value of the PSP stuff was a bit more than the PCE stuff, so it was a good deal at the time.
I did eventually get some more PCE hardware, I currently have a US TG16 CD setup and a full PCE briefcase, but Dracula X and Lords of Thunder cost way too much now for me to grab. Same with the Duo, no way am I going to get another US system, but I may grab a JP Duo sometime if I can find it cheap. Thank goodness I held onto my Gates of Thunder 4 in 1 disc.
I know that's not Neo related, but meh. I sold most of my MVS carts last year, and several home carts this year. I don't miss a thing, and the freed space allowed me room to set up a badass game area.