The short answer: Do Not Buy A Used Crib.
The rules for how cribs must be made took effect on June 28, 2011 and essentially made most cribs sold before that date obsolete and, technically, unsafe. After hundreds of incidents, millions of recalled cribs, and an estimated three dozen deaths, the government stepped in and said that cribs were not being made to a standard that parents could count on.
Drop-side cribs, which had been the most common type, were determined to be particularly dangerous. They can no longer be sold in the U.S. You will see them at yard sales, though, but they are no more legal there than at a furniture store. Also, crib hardware and how cribs are assembled have been big problems - problems that are more pronounced with used cribs. Never buy a crib bumper pad, which the American Academy of Pediatrics says can put a baby at risk for suffocation and other hazardous injuries.
So, buy a new crib that meets the new standards. And, if you can't afford one, safety advocates say a new portable crib - which are less expensive - is preferable to getting a used crib.