I think I got what you all may have in mind.
There is a glottal stop between on and někdo. It is necessary to separate the final N in the first word and the starting N in the next word. It is NOT phonemic. A Czech ear does not intepret it as ʔ, ə, or e or uh or however you may call it. It just how we separate sounds that do matter.
Without this sound you would get "oněkdy" instead of "on někdy".
Hehe! I came here to complain that before 2nd coffee o'clock I have an irresistible tendency to think of ‘on’ as referring to either an indeterminate subject and/or to first person plural and/or any of the many other usages of French ‘on’.
The fact that both in Czech and French it is conjugated as a 3rd person singular pronoun and gives very similar sentence structures does not exactly help with avoiding interference (I am a fluent French speaker).
Hledejte na webu a najdete dobrá vysvětlení https://www.helpforenglish.cz/article/2012082801-sometimes-vs-sometime
No, the point is what constutes the new information in the message, the thing that you want to inforrm about and that is the word "kupuje". You know about the milk already but you are stressing he BUYS it here or that he DOES BUY it here. It is the comment.