Can a single man buy a used mattress from a single woman? Is there any halachic (e.g. concerning modesty, I know that there are specific prohibitions for single men in halacha) issues that I am not aware of?
Rabbi Azriel Chaim Goldfein Zatzal told me once that we do not prohibit things that are not mentioned that they are prohibited. To the best of my knowledge there is no such prohibition.