Indeed healthy people
is the object of the making
while it becomes the subject for to buy
All you need ask for subjects is who/what does the action while for objects you ask who(m)/what receives the direct action. It is perfectly alright to have healthy people
be both. In this case though people is neither the subject nor the object of an actual conjugated verb but rather of 2 unconjugated actions, which is fine (the object of an 'ing'/present participle modifier and the subject of an infinitive). So indeed also the infinitive phrase is the object of the adjective (less likely) and modifies it in this case.
Was there something specific that confused you here?
Economist GMAT Tutor