I don't understand the aversion to used safety gear.
Gear - Likelihood of using to protect someone - Likelihood of failure if used - Risk
Car seat - Low - Low - Low
Bike helmet - Low - Low if not excessively old or damaged - Low
Motorcycle helmet - Moderate - Low if not excessively old or damaged - Somewhat low
Climbing gear - High - Low if it can be inspected (like metal gear or simple webbing. Ropes are harder to inspect since you can't see the inner strands) - Moderate at most
People won't use used car seats, but they will buy cars with used seatbelts, and lots of other safety equipment that's "used" but probably hasn't actually been in an accident? There's no difference. As long as the car seat is a safe design, and it has no damage, it's probably fine.
For bike helmets, people rent these sometimes. They can be cleaned (I've normally seen Lysol spray* used with no adverse effects) and inspected. It's surprisingly hard to break a bike helmet, even after five years. A 1lb hammer won't do it, in my experience, and I've retired a few old and new helmets. A crash might be grounds for retiring it, but even then there's still a lot of protection left unless it has major damage. Motorcycle helmets might be a bit different since they have to withstand greater impacts.
For climbing gear, metal gear generally doesn't get hidden damage that will cause it to suddenly fail. As far as I know, carabiners dropped on the ground are fine to use, as are ones used to take normal falls (one used for towing a truck would not be OK, but it probably wouldn't work right since it would probably be deformed). There's a huge margin of safety on all this stuff.
For used shoes, Tinactin makes an antifungal spray, but I think that might just inhibit growth, not kill the fungus. I would try something like hydrogen peroxide (though that might bleach or damage fabrics, and I'm not sure if it's an EPA registered disinfectant), a quaternary ammonium disinfectant like benzalkonium choride, or hot water that's not hot enough to melt the shoes. Alcohol like Lysol spray is good for taking care of bacteria, and it might have some effect on fungus. See here for a procedure to kill fungus that causes disease in bats:
https://www.whitenosesyndrome.org/sites/default/files/resource/national_wns_revise_final_6.25.12.pdfBasically, hot water is the best way to disinfect anything. Any shoes that can get wet can surely withstand 50C water, and you could increase it to 60C in case foot fungus is more resilient. Almost nothing survives over 60C.
The same principles work for underwear (though I haven't seen a lot of used underwear for sale). Wash it with bleach and/or dry it on high heat, and it will be completely sanitized.
I have a sonicare toothbrush, and it grows mold inside the brush head where it screws onto the handle. I clean it with bleach added to boiling water. The handle is dishwasher safe, and the heated dry cycle on the dishwasher sanitizes things. You can even use it to cook fish.
http://www.foodnetwork.com/recipes/dishwasher-salmon-with-a-piquant-dill-sauce-recipe.htmlBleach eventually breaks down into salt water, and at low concentrations it's similar to the chlorine that's already in municipal water. It's highly corrosive at normal concentrations, but after you rinse it off a little residue isn't harmful. You can use a small amount of bleach to disinfect contaminated water (look up the correct amount before you do).
*note that Lysol makes many products, but as far as I know, the aerosol spray is mainly alcohol and perfume.