This - also buy mattresses new however our new Moses mattress was around £11 and a new mattress for the next2me was £21 (both Amazon) so as long as it has the right certifications it doesn’t have to be expensive. Babies do not need soft mattresses, they need a firm, flat surface. Cot bumpers and quilts etc are dangerous too. People spend a fortunes on crap to put in a crib which increases the chance of SIDS A sheet is all you need.Second hand for baby is fine for a lot of stuff except car seats. It isn't recommended to use them second hand. They also have a best by date.
Also, if you go for a second hand cot, just check it’s got the right certification too.