I think the issue with mattresses is that they're impossible to clean completely. Even from one baby to the next, it's a good idea to get a new one. The way I figure it, if your first baby was sick (and I'm not talking spit-up, but fevers, flus, etc), those germs could still be lurking in the mattress. Why expose the new baby to all that right from day 1? Plus all the dander from dust mites that could potentially be lurking in the mattress (even the plastic coated mattresses can develop little holes, and plastic breaks down over time). Plus, every time a mattress is used, it's getting less and less firm.
This is interesting:
"The study compared 131 babies who died of SIDS in Scotland between 1996 and 2000, and 278 healthy infants. It was based on a questionnaire that controlled for the effects of socioeconomic status and other factors that could affect the risk of sudden infant death.
Overall, there was a threefold increase in SIDS deaths associated with a used infant mattress. A more detailed analysis of the questionnaires found that the risk was greatest if the mattress came from another home, the study says.
This is the second study to show an association between a used mattress and SIDS, Tappin says. He was involved in the previous study, which was published in 1997."
https://preventdisease.com/news/articles/used_mattress_cause_sids.shtml
I get that babies are expensive and money can get tight, but I don't know. You can buy a new mattress for like, $100. There are so many other ways to save money, why nitpick over something that may increase the risk of SIDS?