I like to consider a few questions to help me decide if I should keep, sell, donate, or trash something.

1. In what scenarios will this be useful to me?
2. How likely are these scenarios to come up?
3. If it does come up, will I remember I have this and be able to find it?
4. What other options will likely be available?
5. If I did get rid of this but realized I need another one, how difficult or expensive will it be to replace it?
6. Given how much space it takes and/or how much it will be in the way and the above responses, is it worth keeping? I'll also consider how much space I have, like if it takes up space in a toolbox where I still have space available, the "keep" option is a lot more likely than if I need to move things around each time I open it just to get it closed again.
7. (Once I've decided to get rid of it) How much value does it have to others?
8. Could anyone I know use it?
9. Would anyone I don't know want it?