Are my prayers invalid because I did not buy the ointment at the time, knowing that there were traces of feces?

Your past prayers, performed while experiencing سلس (incontinence), are valid, even if you did not treat your condition with an ointment. This is because, according to some Maliki jurists, treating incontinence is not obligatory, due to the uncertainty of the treatment's efficacy. Based on this opinion, your ablution and prayers are valid, and you are not required to repeat them.