Just wondering exactly where I can find the scripture that supposedly tells us we can’t buy or sell on the Sabbath.
While you will not find anything in Torah that specifically addresses "buying or selling" on the Sabbath, it was clearly understood by Israel as being taboo. Let's check Nehemiah 10 where we see the people of Israel swearing an oath that included NO COMMERCE on Shabbat:
Nehemiah 10: 28 The rest of the people, the cohanim, the L'vi'im, the gatekeepers, the singers, the temple servants and all who had separated themselves from the peoples of the lands to the Torah of God, along with their wives, sons and daughters, everyone capable of knowing and understanding, 29 joined their kinsmen and their leaders in swearing an oath, accompanied by a curse [in case of noncompliance], as follows: "We will live by God's Torah, given by Moshe the servant of God, and will perform and obey all the mitzvot, rulings and laws of ADONAI our Lord. 30 "We will not give our daughters as wives to the peoples of the land or take their daughters as wives for our sons.
31 "If the peoples of the lands bring merchandise or food to sell on Shabbat, we will not buy from them on Shabbat or on a holy day. "We will forego [planting and harvesting our fields] during the seventh year and collecting debts then. 32 "We will impose on ourselves a yearly tax of one-third of a shekel [one-seventh of an ounce of silver] for the service of the house of our 33 for the showbread, for the regular grain offering, for the regular burnt offering, for [the offerings] on Shabbat, on Rosh-Hodesh, at the designated times and at other holy times, for the sin offerings to make atonement for Isra'el, and for all the work connected with the house of our God.
34 "We, the cohanim, the L'vi'im and the people, will cast lots in connection with the wood offering, so that it will be brought to the house of our God according to our fathers' clans, at specified times, year by year, and then be burned on the altar of ADONAI our God, as prescribed in the Torah. 35 "Every year we will bring the firstfruits of our land and the firstfruits of all fruit from every kind of tree to the house of ADONAI. 36 "We will also bring the firstborn of our sons and of our livestock, as prescribed in the Torah, and the firstborn of our herds and flocks, to the house of our God, to the cohanim ministering in the house of our God.
37 "We will bring the first of our dough, our contributions, the fruit of every kind of tree, wine and olive oil to the cohanim in the storerooms of the house of our God, along with the tenths from our land for the L'vi'im; since they, the L'vi'im, take the tenths in all the cities where we farm. 38 The cohen the descendant of Aharon is to be with the L'vi'im when the L'vi'im take tenths. The L'vi'im will bring the tenth of the tenth to the house of our God, to the storerooms for supplies. 39 For the people of Isra'el and the descendants of Levi are to bring the contribution of grain, wine and olive oil to the rooms where the equipment for the sanctuary, the ministering cohanim, the gatekeepers and the singers are. We will not abandon the house of our God." (CJB)
Pay special attention to verse 29, where we see that the people are talking about entering into an oath/vow, to walk in Torah. The things listed that they were vowing to change included the fact that they would no longer buy nor sell anything on the Sabbath!
The above passage (along with Nehemiah 13:15-21, below,) are very clear indicators revealing that Israel KNEW that buying and selling on the Sabbath were breaking ADONAI's divine rules.
Let's take a look at another passage, that discusses the prohibition against commerce on Shabbat:
Nehemiah 13:15 During this time I saw in Y'hudah some people who were treading winepresses on Shabbat, also bringing in heaps of grain and loading donkeys with it, likewise wine, grapes, figs and all kinds of loads; and they were bringing them into Yerushalayim on the day of Shabbat. On the day when they were planning to sell the food, I warned them not to.
16 There were also living there people from Tzor who brought in fish and all kinds of goods, and sold them on Shabbat to the people in Y'hudah and even in Yerushalayim.
17 I disputed with the nobles of Y'hudah, demanding of them, "What is this terrible thing you are doing, profaning the day of Shabbat? 18 Didn't your ancestors do this, and didn't our God bring all this disaster on us and on this city? Yet you are bringing still more fury against Isra'el by profaning Shabbat!"
19 So when the gates of Yerushalayim began to grow dark before Shabbat, I ordered that the doors be shut; and I ordered that they not be reopened until after Shabbat. I put some of my servants in charge of the gates, to see to it that no loads be brought in on Shabbat.
20 The merchants and sellers of all kinds of goods spent the night outside Yerushalayim once or twice, 21 until I warned them, "Why are you spending the night by the wall? Do it again, and I'll use force against you!" From then on they stopped coming on Shabbat. (CJB)