How is sabja different from chia and does that affect where I can sell it?

They are different plants and different markets. Sabja is the seed of sweet basil, it is black, swells fast in water, and is the traditional seed used in falooda, sherbet and rooh afza type drinks across India and the Middle East. Chia is from a different plant, the seeds are grey to brown with spots, and it is marketed mainly as a modern western superfood. Because the markets differ, you should sell sabja into the drink, falooda and traditional health trade where it is well known, and not assume it will fetch the chia superfood price. Mixing or mislabelling the two can get your lot rejected by a serious buyer. Know which one you are growing and sell it into the right channel.