Could you say 'τα μήλα που αγόρασα' ?
Yes, you can. ;)
We could also say : "the apples I bought"
Yes, that is one of the accepted translation.
Unfortunately, in English, when you forget an "s" in "apples", your sentence is completely wrong when it's actually a typo.
No way to tell whether it was a mistake or a typo. Just better get used to proofread my answers!
Typically responses like that are not marked as typos if they change the meaning. Forgetting an "s" there creates a singular apple. If you put something like "appies" it would likely be just considered a typo.