Jäätelokone

*Imperfect tense and genitive since it happened before and you did the action

**This word can be left out. Especially since later you use "joku" again which doesn't give the impression that it is necessarily the same person buying the machine. It isn't wrong, but it sounds a bit silly to repeat the word "joku" to me.

***You are selling the thing in tori.fi so the ending is -ssa or -ssä depending which is more natural ending to say. I don't remember what this case was called. Examples like autossa, korissa, kaupassa, hississä.

****Since the part belongs to the ice cream machine, the "kone" is conjugated as "koneeseen" which I think is illative case.

*****I don't understand this part, sorry. If you wanted to say that you included the missing part after finding it, you could say something like "löysin osan ja lisäsin sen pakettiin/korjasin koneen". If that was the case, then I would move this sentence before the apologizing.

In the last sentence, I would specify that you send the ice cream machine to that somebody. It wasn't really wrong, but this way it is more clear in my opinion. Or you could just change the whole sentence to "Lähetän jäätelökoneen hänelle postilla".

Sorry if the explanation is messy.

Jäätelokone

Myin jäätelökoneen jollekulle/jollekin Tori.fi:ssä. Mutta unohdin (lähettää) osan, joka kuuluu koneeseen. Pyysin anteeksi koneen ostajalta, joka ymmärsi, ja asia on kunnossa. Menen postiin ja lähetän osan (postissa) (ostajalle).