I think the problem is that it is sometimes quite hard to translate something that does not exist in the same way in another language (locative case).
In "I have (the) sandwiches.", "I" is the subject and the sandwiches are the (accusative) object of the sentence.
In "Sandviçler bende." the sandwiches are the subject, and "I" is the (locative) object.
So I think that the most literal translation would be something like "The sandwiches are with/on/at/in me". (Note that you need the "The" here, because omitting it would make you the place where sandwiches usually are found. Kind of the natural sandwich habitat.)
That is, because if a word is the subject of a sentence, it usually is definitive too, using the article "the" in English.
So I think this is the reason why the sentence has to be translated with "the sandwiches" and not just "sandwiches".
I agree with you.. I also think that "I have sandwiches" is wrong in this case because the phrase "sandviçler bende" can be used mainly as an answer to "sandviçler nerede?" and not "sandviçlerin var mı?".. thus the answer can not be "i have sandwiches" but "I have the sandwiches".. that's at least my viewpoint.. can be wrong, obviously ;)
My answer to this was "The sandwiches are mine" and it was marked wrong which is fine. The correct answer given was " The sandwiches are on me". In the "discussion" the correct answer was given is "I have the sandwiches". This is very confusing. As an English teacher these sentences have entirely different meaning.
General question for this exercise... In my brain it seems to make more sense to associate "bende/sende/onda/etc" with "is/are with me/you/him/etc" rather than "I/you/him/etc have the (...)" I hope I could explain it clearly. I'm basically wondering what the best translation is. In this particular case for example I wrote "the sandwiches are with me" rather than "I have the sandwiches". It was accepted but I would like to know which is a more accurate translation. Thank you!