Lost one side of AirPods, buy another side to complete the pair
TLDR: Yes it's quite possible a used AirPod (only risk is incompatibility) can be paired to your current AirPod by following these directions (quoted below).



Apple's If your AirPods are lost article is a good starting point (originally linked by IconDaemon).

Before seeking a replacement, make sure you've attempted to find the missing AirPod using Find My iPhone.

If you need to replace it, Apple recommends you "get the serial number and then contact Apple Support for a replacement" - presumably to make sure their versions are compatible.

Regarding setup, you can check out Set up your replacement AirPods or charging case:


  
  Put both AirPods in your charging case.
  Open the lid and check the status light. It should flash amber.
  Press and hold the setup button on the back of the case for 5 seconds. The status light should flash white. If it still flashes
  amber, connect the case to power, close the lid, and wait 10 minutes.
  On your iPhone, go to the Home screen.
  Open the case—with your AirPods inside—and hold it next to your iPhone. A setup animation appears on your iPhone.
  Tap Connect, then tap Done.
  


If you're interested in buying a replacement directly from Apple, you can find more information under AirPods Repair (note that this is US specific, you can find the locale specific page here), where they list a replacement of a lost AirPod as costing $69 + tax + $6.95 shipping fee.



I personally would not want to use somebody's used AirPod, but if necessary I would clean out the earwax and wipe it down with a tissue soaked in hand sanitizer.