I know lots of people that buy second hand gear, especially cams (they're expensive!). I'm pretty sure you'll find there is a good market on ebay, etc for stuff like this.
Climbing.com offers some good advice on what to check:
Any bend in a cam’s axle compromises the unit’s strength, even if the
spring and trigger actions are still smooth. Regularly inspect your
cams to make sure the axles are in good condition. As with biner
gates, it’s essential that a cam’s springs work well enough to hold
the unit in place until the cam is weighted.
If the springs are sticky, give the device a good cleaning. Cam lobes
slightly deform where the metal touches the rock during a fall. Small
dents and deformations are fine, but any significant deformation can
prevent a cam from rolling smoothly when loading. In this case,
replace the unit. Most cam manufacturers will replace the slings for a
reasonable fee, as will several third-party stitching services —
follow the same guidelines for these slings as you would other
Main issue with cams is the sling. I'm not sure I'd trust a second hand cam sling, but given that a new cam can cost £50 I'd buy a second hand one for much less and replace the sling, I'd probably still save money on this.
Take a few good falls onto a nut placed in a horizontal and the cable
will likely fray. After a good whipper, inspect the nut to make sure
there aren’t any sharp burrs or frays in the non-weight-bearing ends
of the wires that could tangle or damage your runners or draws —
remove these burrs with a standard file or nail clippers. If there are
frays or burrs in the weight-bearing wires, replace the nut. In the
case of micro-nuts, especially the silver-soldered variety, bent or
damaged wires at the head (from falls or aggressive cleaning) are
common, and are grounds for immediate retirement.
Your right, second hand ropes and slings, etc. should prob be trashed.