Ref. No. 2111/44-2186
In the name of Allah the most Gracious the most Merciful
The answer to your question is as follows:
The animals slaughtered according to the Shariah method got tanned and are pure, and the animals slaughtered in the non-Shariah ways must be tanned to get purified. Until they are purified, their sale is not permissible. If the people, from where and whom you buy the skins tell you that it is impure then it is not allowable to sell it, but if it is not known at all and no major guess can be made, then it is considered to be pure. It will be permitted to act as an intermediary in buying and selling it and your earnings will be lawful.
All kinds of skins except pigs’ can be sold after tanning, just as it is tanned with drugs and clay, it can also be tanned by just sprinkling salt. If you don't tan the skins at all, they would be at high risk of damage.
And Allah knows best
Darul Ifta
Darul Uloom Waqf Deoband