Just buy a new mattress, as you need one that hasn't been indented. We got ours of a friend, it is 7yrs old. However we didn't need a new mattress as it is a specially designed non-denting mattress. Our cot we bought of a different friend and it is about 20-30 years old. We bought a brand new mattress for it.
I can only second what everyone else has said. I would buy a new mattress. Not sure how well they tolerate being washed? They are in them for such a small amount of time anyway (mine only lasted 6 weeks) I can't see anything wrong with using it.
The bassinet I'm using is about 38 years old. It was my sister's, then my two cousins, then me and my brother, then my niece and nephew and now my LO. I had to get a custom made mattress because they don't make the right size any more!
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