I've done a lot of things I "shouldn't", so it's how comfortable you are with not following the rules. It's a sweet little baby so I bet you want to be pretty careful within reason. We had a ton of used baby stuff. I would either wash it or dry it depending on the item to try and kill anything. For our crib mattress I had a used one I had been saving forever up in the rafters of my attic. When I went to pull it down I found it had mold growing on it. I could have just washed the exterior, but I threw it out and used a new one. If it hadn't had mold I would have used it.
My child has the best immune system of her peers, she gets a tiny cold about 2xs a year. She crawled on floors with dogs and cats, ate food that fell on the floors, has used clothes, toys and bedding. I don't care if she shares food with peers at school, I figure the more germs the better. Our toddler bed is a used one from craigslist for $20. Use your best judgement on used stuff, and remember how hysterical we are as a society of germs.
I've never heard of sids from a used mattress (as the other responder mentions), I'd love to see the link to the research. We also co-slept the first 4 months which you "shouldn't" do, so that used mattress would have been less of a problem if it had worked out. Congratulations on your baby, I LOVED the first few months, even with the lack of sleep.