Because they were being labeled as insurrectionists to the roman empire.
So... why make it look like the label is correct by selling their clothing to buy weapons?
Answer the question. What were the swords for? Peter is the only one who used it, and his action was immediately undone. The swords that Jesus spent his precious breath ordering his followers to sell their clothing for are never mentioned again. Why?
I think this is just like any other legend/mythical figure. Jesus of Nazareth was an insurrectionist, starting a new religion based on John the Baptist's teachings. He brought a mob to Jerusalem, and got them armed. He was rousing the rabble, threatening the Pharisees power, and so they pressured the Romans into killing Jesus before he could start a revolution. He was made an example of, very publicly and brutally- "This is what happens to a man when they challenge the (((status quo))), flogged to ribbons, mocked and humiliated, and left crying out for their father as they're nailed to a cross".
Over the mists of millennia, Jesus's legend grew, and decades after these events actually happened, people started writing down the oral accounts of what happened, spicing it up with miraculous powers and such. These stories floated around for hundreds of years, before King James came along and compiled them all into one story, heavily edited to remove contradictions and try to align timelines as best they could.
I'll answer this directly...
Jesus made them buy swords in order to show them how silly they were.
They had them, but soon realized they were powerless against the entirety of the Roman army, as well as the Jewish elite.
Jesus then showed them the rest of the plan...
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