All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
There is no harm on you in selling fabric (textile) to the people. If they use it in a forbidden manner, then the sin falls on them and not on you. Allah Says (what means): {…and no bearer of burdens will bear the burden of another.} [Quran 6:164]
It should be noted that if wearing clothes below the ankle is not done for pride (and showing off), then this is an issue of difference of opinion among the scholars.
Ibn Qudaamah said: “It is disliked to wear the Qamees (grament), the Izaar and trousers below the ankle as the Prophet commanded to raise the Izaar above the ankle. However, doing so out of pride is forbidden.” [End of quote]
Ash-Shawkaani said in Nayl Al-Awtaar: “The Prophet said to Abu Bakr: 'You are not one of those who do so conceitedly (i.e. you do not trail the garment arrogantly).'; this is an explicit statement that the prohibition is related to “conceit” (and arrogance).” [End of quote]
Therefore, not all people who do so (drag their clothes below the ankle), do it for showing off and out of pride.
It should also be noted that helping in what is forbidden, if it is an indirect and an intentional help, then the person is not sinful, such as is the case when one sells items that have both lawful and unlawful uses, and does not intend to help the buyers to commit prohibited acts with them. Another example is when someone gives another a Dirham (silver currency) to buy something other than wine but he goes and buys wine and drinks it; then the person who gave the Dirham does not bear a sin for giving it if he did not intend to help him (the other) commit a prohibited act.
In any case, there is no harm on you in trading in fabric and selling it to the people.
For more benefit, please refer to Fatwa 321916.
Allah knows best.