I have a foundational question about tevel. It says in Bava Metzia 88b that consumption of purchased tevel is not forbidden Bibically, but rather only Rabbinically. It seems all the major rishonim agree. For instance, Rambam writes in Hilchos Maaser 2:2 that as long as bought after the gmar melacha it is only D'Rabbanan (based on the pasuk that says "tevuas zarecha" meaning "the produce of YOUR crops" that YOU GREW (not bought). Others like the Ravad and Kesef Mishnah concur and go further and say that it is only D'Rabbanan whenever purchased. I am not aware of ANY dissenting view that holds eating bought tevel is an issur d'orysa.
We know the punishment for willfully eating tevel is misa b'dei shamayim. But if eating bought tevel is only an issur d'rabbanan, then would the punishment for eating bought tevel still be misa b'dei shamayim? Can there be a punishment of misa b'dei shamayim for an issur d'rabbanan? And more fundamentally, so much of seder zeraim is focused on the seriousness of eating tevel or buying tevel. The whole masechta of demai is based on the concern that there is a small chance that produce sold to you by an am ha'aretz might not have been tithed. But if eating bought tevel even intentionally is only a d'rabbanan, why the great concern? And when we assume that amei ha'aretz always at least take terumah gedolah because they respect the gravity of selling produce that contains terumah gedolah which carries the penalty of misa b'dei shamayim - well...not quite if eating bought tevel is not even an issur d'orysa?
This question cuts to many foundational elements of seder zeraim and I am hopeful that someone might be able to shed some light on this? Thank you!