I wouldn't buy second hand shoes for myself, but I have bought my daughter two pairs of second hand shoes. I have also put hand-me-down shoes on her. I do run then all through a high temperature cycle in the washing machine, and I only put her in Clarks shoes since she has pairs I have bought there and I get her feet checked every month so I know they definitely fit her. I buy them from a local baby market and I only buy them from clean/well presented stalls and only if the shoes have clearly been barely worn (as in, no scuffs or marks, no visible wear on the soles)
My issue is I can only get shoes to fit her in Clarks, she is too small for most places, and all their shoes are garish so don't go with most stuff!