I was given a second hand one also - I was slightly concerned but I sprayed with a water/lemon juice solution and left it in the sun to kill any bacteria. it has a plastic sheet then a sheepskin and then the sheet on it - so I really don't think there will be any issues with it.

I personally don't think there is any problems with second or third hand mattresses - I checked mine for stains or dark spots and anything that I thought looked suss I sprayed 100% lemon juice on it because it's a natural bleach.

I understand what you're saying though - new would be better for piece of mind - but honestly will your babe ever be on the mattress with nothing between him/her? probably not.

hope that helps a bit