Do people have to ship the clothing to you? If they have out of pocket costs to donate, they probably won`t.
They might also prefer to donate to a non-profit thrift store rather than give things to someone who will be making money by reselling something they got for free.
If none of the above is the case, offer to pick up everything that`s local to you. People are inclined to want to give stuff away but not so inclined if they have to deliver it.
Just a couple of thoughts............