So so so ridiculously brittle, I think I've only had one or two that were completely intact, and more than a dozen have passed through my hands in the past few years. Even the blue ones are starting to deteriorate...
I put my boxed Atari ST on eBay when I was moving house about 15 years ago. It came with the Powerpack, over 100 boxed games in fantastic condition, (including all the ST Lucasarts titles) a 30mb Atari HDD, every copy of ST Format up until issue 44, and about 300 loose floppy disks, cover disks, PD, and a few pirated games. The whole lot went for £10. I should have cancelled the auction, but my thinking at the time was, well it's going to a good home (It was, most of the ST Format Scans come from my copies of the mags, same with the disks, so not that bitter about it, it really did go to a good home. Just pissed off that I ended up buying another one a few years back with nothing apart from the computer with a modded sdcard floppydrive and it cost me over 100 quid. I also regret selling my Nomad that had SF2, all the sonic games, Virtua Racer, Gunstar Heroes, Terminator and the Sega Smash Packs for £20. I sold it in 2001 because I didn't like that it only played in Black and White on PAL TV's
I regret pawning my (five) Sega CD games and my ten or so 3DO games. These included CIBs for Final Fight CD, Sonic CD, SSF2T, Need for Speed, and a few other nice titles. I regret throwing away (>.<) a snes, genesis, saturn, dreamcast, and maybe ps2. And I regret trading in my 3DO.
I traded in my whole collection of PS1 games (including every launch title, all the FF's and all 5 of the RE's and my Launch edition PS1 (still boxed) on launch day for a PS2 and TTT and RR5. Ended up buying back most of the PS1 games within the year from the same place, most of them were still my copies but mysteriously almost triple what I traded them for even though the store had a policy of "We give you 50% value for trade"
i sold my collection of game gear games along with the console when i was about 15 for £20. sad sad times. its prob not worth much more now but its the sentimental value.
I traded in my Neo Geo Pocket collection for a mint, boxed Jaguar and 5 games. It seemed to make sense at the time, I wasn't playing the NGPC any more and I wanted Tempest, Raiden, Cannon Fodder, Rayman, Alien vs. Predator etc. Looking back it was utterly foolish. I had Faselei, Evolution, Cotton, and about 10 or 11 other quality titles. All together it would be worth four figures now. Feels good to vent about it though, we've all made mistakes.
Selling a few N64 carts, one of them was Mario Tennis. I wanted OoT and GoldenEye 007 back then, but for some reason the money I had was only enough to rent them for a while. It wasn't until late 2006 when I got the former via a friend who had his machine no longer working.
clearly, my boxed Jaguar CD, my Xbox DevKit, and my SNSYS GBA stuff - all sold in 2009 for too little money. I am actively looking for a good priced Jag CD to recomplete my home console collection since a few years, but they are tough to find at reasonable price (300USD loose, 400USD boxed).
I have sold Dev systems, test equipment, unique signage, internal paperwork, rare promotional items, etc.....so far I have no regrets as far as missing the things I've sold. As far as wishing I'd held off a few years and gotten more for them, that's another story!
I sold all of my collection. Biggest regret was selling the Dreamcast Loop checker disc and another similar DC test disc for probably way less than they were worth.
Years ago, changed my Mega CD games, Final Fight and Sonic CD for two Ps1 Games... Shadowman and Time Crisis. Also sold my Zelda Game&Watch for 7€. It still hurts.
My little story: I sold an Amiga 1200 (with a blizzard 030 accelerator, 32mb Ram, CD-rom drive, Commodore Monitor and HEAP LOADS of big box games) for about £120 back in 2000. Spent the money on a cheap GoldStar DVD player, which was shit! Sold that soon after and got a couple of Dreamcast games; Crazy Taxi and Soul Calibre i think. Loved those games, but I wish I had my Amiga games back. Will the pain ever go away?