I'm sorry your SD is going through this. However, I'm going to guess that the truth is not EXACTLY what your SD is saying.
Like another poster said, who wants used underwear? That's not something you would sell. MAYBE her mom would sell a winter coat if it was a designer one, or shoes if they were designer.
I do know my SD was NEVER good about bringing her clothes back and around age 7-8 her mom stopped making sure she did. That's when the clothes would disappear. My SD's mom is not vindictive, she did not sell the clothes, they just disappeared because my SD was not responsible enough to keep track of them and her mom couldn't be bothered.
My SD's mom is not good about buying her underwear, and my SD is not good at reminding her mom at a good time. She'll say something about it at a moment when my SD's mom is super busy, or stressed. Then she's likely to get a "I'm not going to get it for you now" type of response.
If I were you, I'd buy nice, but cheap or generic jackets to send with her. Even go to the thrift store. Send cheap but clean underwear. I got my SD a jacket for $4 at the thrift store.
I honestly don't believe your SD 100%. Her mom may be neglectful, but I'm guessing your SD isn't being as responsible as she could be either.
I will also mention that there was a coat that my SD had that her bio mom said she hated. One day we sent her to her mom's in the coat and then when I picked her up, she didn't have it. My SD looked all over for it, and her mom swore that she didn't know where it was. My SD told us that her mom threw it away because she didn't like the coat and WE BELIEVED IT.
3 weeks later she went to a friends' house, and they told us that she had left her coat there. So my SD's mom DIDN'T throw the coat away, and my SD lied.
Lots of times Stepkids can exaggerate the truth, or say things they think you will sympathize with. I've noticed this with my own SD.
At any rate, you can buy her some nice things, but go cheap and plain. Teach her how to be more responsible with them. You can even provide a dirty clothes bag for her and she can put the dirty underwear in it and bring it to your house to wash. That's a solution we found that works as well for disappearing clothes.
Good luck!