The one component that you wish you had not bought

Wadia WT2000 transport.  It randomly wouldn't read CDs.  Sounded great when it worked but it was God awful expensive and temperamental.

Fritz Carbon 7 monitors.   This will really stir it up, Bob Marley style, but the stock version is just not very good.  I'm prepared for crusifixtion but the stock version at moderate or high volumes is strident, hard, unpalatable to even a marginally savvy acoustically sensitive ear.   This is not what quality audio is supposed to sound like, of course in MY opinion.  In read all the reviews, swallowed the talk, really looked forward to their arrival, and even Mrs. Boss said, 'you bought them why?'  Average cabinet construction, simple little silicone caulk glued in X-over, basic speaker cable terminals, all in all a very unimpressive package. Sonically they lasted about a week.   The good news? Good drivers; Scanspeak.  Had a pro speaker designer construct some decent crossovers, added some damping and they now sound good. But the stock product? Fuggetaboutit! 

I guess I'm pretty lucky- except for a few cheapie phono cartridges I bought in the 1970's before I decided to buy a Shure V15 Type 3, I've bought no turds. 

The early non-async versions of the Wavelength Brick USB DAC and the Wavelength Cosecant USB DAC. Unfortunately it took me 2 Wavelength DACs to figure out Gordon Rankin made some inferior DACs and making them tube based probably hurt them more than it improved the sound.   

In the 40+ years I've been serious about listening to music, there have been many component I have bought and haven't kept. Mostly because I live in a rural area and most of the component were bought without listening....This is from the last 10-15 years.
What I have regretted buying...Cambridge 640 CD Player....Musical Fidelity A3.2, Linn Klyde cartridge. Lehmann BlackBox, Spendor Ls3-5,
Best buys have been...Denon DL301, Naim 150x combo CD5X Flatcap, Whest Phono, Neat Elite, Hercules\Mose, Ittok rewire, 
What I regret selling...Rogers LS3/5A, Supex 900.