Well the way I look at it.

If I have no use for the part at the moment I'll sell it or give away, If its broken I ether fix it or scrap it.
Never toss old pc parts, you can get a little back out of dead parts, steel, copper, aluminum are all worth money. And you save room in the land fills

To answer the title, I have some factors that come to play for me to decide on getting rid of computer components:

1. Does it work? - if the component doesn't work at all no matter how hard I try to fix it and how many systems I test it on, I typically junk it; as far as complete systems in nonfunctional order goes, I just take out as much as I can and get rid of whatever doesn't work anymore.
2. How will it clash in with storage space? - If I have too many parts of the same specification, I either put it into a system that would benefit from it or try to sell it.
3. Will I ever find myself using it on a permanent setting again? - If I don't see myself using a non-valuable part in any system for a long enough time for it to cause a conflict (see factor #2), I sell it as long as I test it again to see if it works, otherwise it gets junked (see factor #1).

In this year alone, I've gotten rid of the following parts for the reasons mentioned:

- S3 Trio 64V2/DX PCI Video card - was supposed to be used on my P1 rig but thorough testing confirmed that the card is no longer in usable condition and not to mention it had a surface scratch near the PCI connector pins, so I've replaced it with a Virge card and the now-dead Trio64 card is being used for parts (managed to get the BIOS chip and two of the RAM chips removed, I think I'll remove the VGA connector to help repair another dead goodie in the future).

- HL-DT-ST PATA DVD-RW Drive - was the original optical drive on my Dell Optiplex GX520 desktop, but unfortunately the lens motors are shot because it has trouble reading discs that doesn't have much data on it, so I've temporarily replaced it with an NEC DVD-RW drive with a beige bezel for until I get around to finding a black bezel DVD-ROM drive, this dead HL-DT-ST drive has been sent for recycling.

- 500W Apevia ATX Power Supply - was used on my AMD64 rig mainly because I desperately needed a PSU to get it back online so I could continue with my gaming adventures due to the original Thermaltake unit biting the dust, the SATA connectors are shot on this Apevia unit (the rest of the PSU worked), the result was this Apevia PSU has been donated to a thrift store and the AMD64 rig is now using an Antec 450W PSU.

Broken not worth fixing and don't need the parts for other rigs gets recycled.
Something I got for free just to try it out and I don't like gets sold, passed on to somebody else, or recycled.
You have to think about is the item something that will be hard to replace down the road if you change your mind.
Anything else gets fixed, cleaned, repaired, and then shelved till needed.

I am pretty picky on snagging things so I don't have too many problems on deciding to keep them. People who buy whole collections or lots would have a harder time.

I have an excess of cases that just don't fit my collection, and even a couple of complete systems that I cut my teeth on early in my retro experience, that I am trying to figure out what to do with myself. One thing I know is that I won't be throwing anything away. I will either sell, trade, or give the excess away.

I will keep an eye on this thread in hopes of being inspired by somebodies solution to storage issues for sure!