I didn't buy any MCN 2nd hand with my son, because I wanted them to last through 2 children and decided new was a better option for that purpose. If I were to need top up numbers of MCN due to expecting twins, I wouldn't have a problem with buying them 2nd hand.
I bought lots of clothing 2nd hand and much of it was barely used for the small baby items. These items are just as likely to have had baby chuck, poo and wee end up on them as MCN, so I don't take issue with them. A sanitising wash does the trick.