I haven't sold any games or systems my parents gave me. I don't think I could bring myself to do it at all :\
CIB hardcase Castlevania: Bloodlines and CIB Gunstar Heroes. I had a decent sized collection I ended up letting go of and started to rebuild and those two sting the most.
My Sega Master System Console and box of 50 Sega Master System Game Cartridges and Sega Card..Parents made me give all that away to my younger cousin...
My Amiga 500 with the rare memory expansion sidecar. Had a full 8MB upgrade. Now that I am itching to play classic Amiga, it's long gone.
I really didn't want to. Bills were piling up and I needed a bit of extra money. I regret not thinking of something else.
I regret selling my Turbo Grafx 16 console, which was in great condition, however I only ever played Blazing Lazers on it, and figured all the good Hu cards would be super expensive so i never looked for 'em
I regret selling ... SEGA Master System mk1 (first release with Hang-On ... on a SEGA card included ) with over 30 games. SEGA Megadrive Altered Beast set and over 30 games (I didn't sell Streets of Rage 2 though - despite not having a console to play it on, I could not bear to part with it. I still have it with the receipt in the box ... 20 Jan 1993!) Nintendo Gameboy (first release with Tetris pack) plus about 20 games with all boxes in perfect condition. I ended up buying all those consoles back from eBay but of course, it's not quite the same as still owning your original stuff. I learned my lesson the hard way and have never sold another console or computer, keeping everything from my Amiga1200 onwards.
Ah, I'm sure many of us on this forum have learned that lesson hard and fast. I'm still in the middle of trying to rebuild my once nearly complete Game Boy hardware collection.
None of it was rare, but I had all Zelda games for handhelds either CIB or new. Even a few others like NES Zelda's, and loose snes cart, and a new copy of Zelda for fds. Also had the guide books, most new, some of the harder to get ones used, but without the included extras torn out like maps and all. My wife hated that I would not let her tear it out, and made her go online for the maps lol... Got hard up for money and sold off almost all of it except my fds copy and the loose carts. Did keep my ds/3ds that are complete, but miss seeing all of those gameboy and gameboy advance boxes in my curio
My Sega Japan internal used Saturn prototype console. Missus bugged me to sell it and soon as I sold I've been regretting since.. T ^ T
Every single game system I traded in to get a new one, starting with the NES and working up to the PS3 and now here I am trying to get them all back and more. and Especially upset about the power glove I traded in..yes, it sucked but it's kind of geek chic now. Although I did beat Mike Tyson's punch out with that damn thing!
Regret selling my Virtual On dual stick controller for the Saturn. Then I again sold my Dreamcast dual stick controller. I never learn.