I traded in my SNES, including games like Megaman X2, Yoshi's Island, and Cybernator, at Funcoland so I could pre-pay for an N64...a system that never once got a game as good as any of those ones I just mentioned.
"Hey SNES fans, what was your favorite game from last year? Chrono Trigger? Super Street Fighter II? Scrambled Valkyrie? Ok, cool, here's 50 nigh on identical Crusin' clones and 100 shitty 3D platformers. Don't worry, we have LOTS of sports game still. Enjoy!"
The only saving grace was that Funco gave STUPID trade in back then, $45 for X2, which was a two year old game that only cost $60 new. So while I feel ripped off by Nintendo, I certainly don't feel bad about the deal I got from Funcoland.
Another nice thing was that when Goldeneye came out I was able to swap my small pile of N64 junk for a Saturn and I've loved that thing ever since. A "bad deal" by the standards of the day, but I actually had fighting games and real Bomberman so, IMO, I won this one outright.
Since then I've only ever sold stuff when I was dead broke (the 90s) or when it totally sucked, like when I wanted a blue PSP so I bought the Madden edition while trading in the pack-in to reduce the price.
EDIT: I also sold my US Duo to a forum member. At that point I had acquired a Duo R, which I like a lot more. While today a working US Duo is a $400 fetish object, to me it was just something I needed for playing PC Engine games. Now I also have an IFU setup in addition to the R. I'm still working with the same collection of Turbo crap that started when I got that Duo. My Turbo/PCE collection has never been 1000 games strong, but it's also ever been liquidated.
I avoid sellers remourse mostly by just not selling anything, and not buying stuff I don't want. I've never sold off a complete set of anything because I've never saw the appeal of wasting thousands of dollars on games I hate. With the exception of SNK systems, most game libraries are about %10 fun and %90 mediocre stuff or worse. By "completing" a game library all you are doing is spending a whole bunch more money to get a ton of crap. The dude with the same system and only the 12 best games has a much better collection, and that's the position I try to aim for.
Also, I was never dumb enough to buy a CDI or a Jag so I never had to go through the heartbreak of dumping it all.