I consider a couple of factors when it comes to dropped gear:
- Some equipment is pretty easy to inspect. A carabiner has one moving part (the gate, possibly a second, if it's a locker). Nuts and hexes have no moving parts.
Cams on the other hand are not easily examined.
- Equipment like non-locking carabiners, nuts, hexes, cams are often redundant. If the carabiner I clipped to a nut fails, that point of protection fails, but hopefully I have something else that catches me before I sustain a ground fall or factor 2 fall.
- Belay devices, belay carabiners, the climbing rope, my harness and my cordelette are not redundant. Most of these (except for the rope) are also pretty cheap.
- Some people are comfortable climbing with dropped gear, others are not. A climbing team is based on trust.
- Did I see what happened to the gear on its way down? Was it a 2000 foot drop, or did it slide down a slab and come to rest on a ledge?
- I am no metallurgist and it is impossible to prove a negative (in this case the claim: "dropped gear will not sustain micro fractures")
Here is how I make my decision whether or not to retire a dropped piece of equipment:
Can I easily inspect it, is it redundant, are the people I climb with OK with climbing on this piece of gear, am I familiar enough with the piece to know it is OK? - I keep it.
Any other situation or if I have just the slightest doubt - I retire it. I won't sell it to anybody else, in fact I destroy it to make sure no unsuspecting person inadvertently uses it.
A couple of final thoughts:
I think it is important to come to terms with the idea that every time you go climbing, you might lose some gear, may it be through dropping or retreat. Talk to your partner about this possibility, and how you will divide the cost.
I don't buy gear from strangers who personally would not climb on the gear any more.