First, you of course have to use good judgement on your part. Selling something at a loss to only go a repurchase for substantially more is unfavorable math.
There's a bit of informal assessment to do when thinking about throwing something onto the trade blanket:
- How useful is this to me? Can I re-purpose it?
- How much can I get for this on resale?
- What am I going to use those funds for?
- Would I need to replace/repurchase this in the future? Will I be able to?
- Will the item be worth more or less in the future if I hold onto it?
If you aren't really constrained for either funds or space, then selling off excess gear makes less sense. You don't need the $$$ to fund projects and you can store the excess away for a rainy day. Most of us aren't in that boat.
Personally, I go through a cycle of testing gear, selling off what doesn't work and then usually plowing those funds back into the same projects. Allows for progress while controlling (to an extent) the out of pocket costs.
That's why I refer to it as gear consolidation - selling an old stock and an unused hand guard to buy a new stock, selling an unused knife for a new / needed flashlight...clearing out the unused excess and turning it into something more needed/desired.
Of course, very often the keep vs. sell assessment often works out as a "naa, I'd better hang onto this", too.